The previously undisclosed capability of the submarines bolsters Israel's deterrence in the event that Iran - an avowed enemy -  develops nuclear weapons in the coming years. At the same time, it complicates efforts by the United States and the United Nations to persuade Iran to abandon its suspected nuclear weapons program.

Two Bush administration officials described the missile modification and an Israeli official confirmed it. All three spoke on condition their names not be used.

The Americans said they were disclosing the information to caution Israel's enemies at a time of heightened tensions in the region and concern over whether Iran's suspected nuclear weapons program can be stopped through negotiations.

After intense pressure led by the U.S., the United Nations' International Atomic Energy Agency last month gave Tehran an Oct. 31 deadline to accept full inspections and prove it has no nuclear weapons program.

Iran denies developing nuclear weapons and says its nuclear program is solely for generating electricity. Iran's leaders are resisting more intrusive inspections, setting the stage for a showdown in the coming weeks.

Beyond Iran, Arab diplomats and U.N. officials said in interviews that Israel's steady enhancement of its secret nuclear arsenal, and U.S. silence about it, inflames Arab desires for similar weapons.

"The presence of a nuclear program in the region that is not under international safeguards gives other countries the spur to develop weapons of mass destruction," Nabil Fahmy, Egypt's ambassador to Washington, said in an interview. "Any future conflict becomes more dangerous."

Late last month, Egypt joined Saudi Arabia and Syria at the U.N. general assembly in criticizing the U.S. and the U.N. for ignoring Israel's weapons of mass destruction while pressuring Iran to give up its nuclear program.

Syrian Foreign Minister Farouk Al-Shara, whose country has been accused by Washington of pursuing chemical and biological weapons, said, "Some quarters selectively choose to level their false accusations at some Arab and Islamic states. . . while simultaneously ignoring the Israeli arsenal of weapons of mass destruction."

Israel refuses to confirm or deny that it possesses nuclear weapons. Intelligence analysts and independent experts have long known that Israel has 100 to 200 sophisticated nuclear weapons.

Israel, India and Pakistan are the only countries with nuclear facilities that have not signed the nuclear Non-Proliferation Treaty, which was initiated in 1968 to stop the spread of nuclear weapons through inspections and sanctions.

Israel forbids outside inspections of its nuclear facilities except for a small research reactor. India and Pakistan, which also have nuclear bombs, prohibit inspections of weapons installations; both countries, however, allow U.N. monitoring of civilian plants.

Iran and Arab states with civilian nuclear programs have signed the nonproliferation treaty. The Arab countries, however, also have refused to agree to tougher inspections because Israel will not sign the overall treaty, U.N. officials said.

"A big source of contention is Israel," a senior official trying to win acceptance of the additional inspections said in an interview. "This is a magnet for other countries to develop nuclear weapons."  Israel and its U.S. backers regard its nuclear weapons as an essential deterrent and centerpiece of the country?s security.

While not acknowledging their nuclear weapons, Israeli officials have promised that they will not be first to 'introduce' such weapons to the Middle East. Israeli and U.S. officials said that means Israel would not launch a first-strike attack using nuclear weapons. They argue that other countries have nothing to fear from Israel's nuclear weapons, while Israel has everything to fear from its neighbors.

Since 1969, Washington has accepted Israel's status as a nuclear power and not tried to persuade the Israelis to curtail the program or sign the nonproliferation treaty. "We tolerate nuclear weapons in Israel for the same reason we tolerate them in Britain and France," a senior Bush administration official said in an interview. "We don't regard Israel as a threat."

In turning a blind eye to Israel's program, U.S. intelligence agencies routinely omit Israel from semi-annual reports required by Congress to identify countries developing weapons of mass destruction and similar listings.

The policy allows U.S. administrations to avoid triggering economic and military sanctions required by Congress against nations pursuing nuclear weapons.

In an effort to protect Israel's nuclear complex and other targets, the Clinton administration even barred the sale of U.S. satellite images of Israel that are better than two meters in resolution, though images with resolutions of one meter are available for other countries.

But the Bush administration's determination to stop Iran from developing nuclear weapons means Israel?s worst kept secret is likely to loom larger in negotiations with Tehran, either as a bargaining chip or an obstacle.

"You are never going to be able to address the Iranian nuclear ambitions or the issues of Egypt?s chemical weapons and possible biological weapons program without bringing Israel?s nuclear program into the mix," Joseph Cirincione, director of the nonproliferation program at the Carnegie Endowment for International Peace in Washington, said in an interview.

Israel is smaller than New Jersey and its population of six million is within reach of missiles from Iran and other neighbors. Israel's land-based nuclear weapons are increasingly vulnerable as Iran improves the accuracy of its long-range missiles.

The strategic alternative was to develop nuclear-armed submarines, which would be almost invulnerable, Robert S. Norris, a nuclear historian at the Natural Resources Defense Council in Washington, said in an interview.

The effort to develop nuclear-armed submarines dates back several years. Israel ordered three specially designed submarines from Germany in the mid-1990s and they were delivered in 1999 and 2000.

The attempt to arm them with nuclear missiles was first disclosed in a book published in June 2002 by the Carnegie Endowment. The Washington Post published an article about the effort a week later.

Interviews in recent weeks with officials in Washington and Tel Aviv provided the first confirmation that Israel can now deliver nuclear weapons from beneath the sea. The U.S. officials said the warheads have been designed for American-supplied Harpoon missiles, which have sea-skimming cruise guidance systems and a normal range of about 80 miles. Harpoons usually have conventional warheads and are common in the arsenals of the U.S. and other countries.

Norris said Israeli engineers would have had to reduce the size of a nuclear weapon to fit the warhead of a Harpoon and alter the missile guidance system to hit land-based targets, both relatively simple tasks for a sophisticated weapons program.

"They have been at it for more than 30 years, so this is something within the realm of capability for Israel?s scientists and engineers," said Norris, who added that the range of the missiles might have been extended, too.

The submerged submarines send missiles to the surface in capsules fired from torpedo tubes. When a capsule reaches the surface, its top blows off and the missile is launched. An Israeli government spokesman, Daniel Seaman, confirmed that the three new submarines carry Harpoon missiles, but he declined to specify the type of warhead.

Israel has 170 miles of coast on the Mediterranean Sea and its submarines are scheduled so at least one is in the water at all times to make sure that Israel has what a former Israeli officer described in an interview as a 'second strike' capability if it is attacked.

The Israeli government rejected requests for interviews with officials from its atomic energy agency and refused to answer questions on nuclear-related matters.

"We don't comment on this issue," Seaman, head of the government press office, said. "We are not going to cooperate on this matter."

The consensus in the U.S. intelligence community and among outside experts is that Israel has the fifth or sixth largest nuclear arsenal in the world.

Under the nonproliferation treaty, five countries are permitted nuclear weapons. Britain has 185, the smallest number among the five, according to the Stockholm International Peace Research Institute. The group estimated Russia has 8,232 weapons, the United States has 7,068, China has 402 and France has 348.

Israel has about double the number of India and Pakistan. North Korea claims to have nuclear weapons, but U.S. intelligence officials are uncertain about whether the claim is true. Estimates of the number have ranged from one or two to six.

In addition to its nuclear arsenal, numerous experts said that Israel has an active chemical weapons program and conducts research on biological weapons. Israel began building a nuclear bomb in the mid-1950s when hostile neighbors surrounded the young country and the Holocaust was fresh in the minds of its leaders.

A secret agreement was signed with the French government in 1956 to help Israel build a plutonium nuclear reactor. France and Israel were natural partners then; they had been allies with Britain in a brief attempt to seize the Suez Canal after Egypt had nationalized it, and had shared concerns about the Soviets and unrest in North Africa.

The reactor site chosen was a rugged and remote corner of the Negev desert, outside the village of Dimona.

The top-secret undertaking was a massive project, with as many as 1,500 Israeli and French workers building the reactor and an extensive underground complex on 14 square miles. French military aircraft secretly flew heavy water, a key element of a plutonium reactor, from Norway to Israel, according to the Federation of American Scientists in Washington.

American U-2 spy planes spotted the construction soon after it began in 1958. Israel initially said it was a textile plant and later a metallurgical research facility. Two years later, U.S. intelligence identified the site as a nuclear reactor and the C.I.A. said it was part of a weapons program, according to documents at the National Archives in Washington.

On Dec. 21, 1960, Prime Minister David Ben Gurion told the Israeli parliament that a nuclear reactor was under construction, but he said it was 'exclusively for peaceful purposes.'

It was the first and last time that an Israeli prime minister made a public statement about Dimona, according to 'Israel and the Bomb,' an authoritative book by Avner Cohen, an Israeli-American scholar.

Soon after taking office in 1961, President John F. Kennedy pressured Israel to permit an inspection of the Dimona site. Ben Gurion agreed and an American team visited the installation that May.

A post-visit U.S. memo said the scientists were 'satisfied that nothing was concealed from them and that the reactor is of the scope and peaceful character previously described to the United States.'

American teams visited Dimona seven times during the 1960s, and reported that they could not find evidence of a weapons program. In June 1967, on the eve of the Six-Day War, Israeli engineers managed to assemble two improvised nuclear devices, an accomplishment that is still classified as secret in Israel, according to an interview with an Israeli with knowledge of the episode.

By early 1968, Carl Duckett, head of the C.I.A. directorate of science and technology, concluded Israel had nuclear weapons, according to testimony he gave to the Nuclear Regulatory Commission in 1974.

Duckett said his assessment was based on conversations with Edward Teller, the father of the hydrogen bomb, who visited Israel several times and supported its nuclear program. Duckett said Richard Helms, director of central intelligence, ordered him not to distribute his conclusions.

In 1969, President Richard M. Nixon struck a deal with Israeli Prime Minister Golda Meir: As long as Israel did not go public with its nuclear weapons program or test weapons openly, the United States would stop its inspections and turn a blind eye to its program, according to U.S. records.

The proof finally surfaced 17 years later. On Oct. 5, 1986, the Sunday Times of London published an article in which a former Dimona technician, Mordechai Vanunu, provided a detailed look at Israel's nuclear weapons program. His cache included diagrams and photographs from inside the Dimona complex, which he said had produced enough plutonium for 100 bombs since it went online in 1964.

To conceal the weapons work from U.S. inspectors, a false wall had been built to hide elevators that descended six stories beneath the desert floor to facilities where plutonium was refined and bomb parts were manufactured, Vanunu said.

Shortly before the article was published, a female agent from Israel's intelligence service lured Vanunu from London to Rome. He was kidnapped and smuggled back to Israel, where he was convicted of treason in a secret trial and sentenced to 18 years in prison.

Vanunu is supposed to be released next year. He has been denied parole because prosecutors say he still has secrets to tell, according to his lawyer and supporters. Meanwhile, Israel was enhancing its ability to launch its nuclear weapons.

The U.S. sold Israel F-15 and F-16 fighter jets, both of which can be used to deliver nuclear bombs or missiles. In the 1960s, the French helped Israel develop its first generation of Jericho missiles and the Israelis built a longer-range Jericho II by the mid-1980s.

The Jericho I and II are equipped with nuclear warheads and satellite photos indicate that many are hidden in limestone caves southeast of Tel Aviv, near the town of Zachariah, which is Hebrew for 'God remembers with vengeance.'

Israel has never openly tested nuclear weapons. Experts said the Israelis have used supercomputers, some supplied by the U.S., to conduct simulations for designing weapons. Components also can be tested using conventional explosives.

"Non-nuclear tests would not be picked up by satellites and other monitoring systems," Gary Milhollin, director of the Wisconsin Project on Nuclear Arms Control in Washington, said in an interview. "You can do a lot in secret and without a nuclear explosion."

Its nuclear program remains shrouded by a policy the Israelis call 'nuclear ambiguity.' The phrase means Israel does not acknowledge its nuclear capability and suffer the accompanying political and economic fallout, yet it gains the benefit of deterrence because other nations know the weapons exist.

Though Israel is the freest country in the Middle East, debating the nuclear program is taboo. The Israeli Atomic Energy Commission is one of the country's most secretive organizations. Its budget is secret and its facilities are off limits and employees face harsh sanctions if they talk about its operations. Even the name of the chief of nuclear security was a secret until two years ago.

A military censor also guards Israel's nuclear secrets. Journalists writing about any security or defense matters must submit articles or broadcast scripts for pre-publication review. The censor, an army general, can block publication or broadcast. Decisions can be appealed to the Supreme Court, but journalists said the government usually prevails.

The restrictions have been relaxed for most topics, but not the nuclear program, several Israeli journalists said. As a result, journalists do not say outright that Israel has nuclear weapons, relying instead on code words and repeating foreign news reports.

Foreign journalists in Israel are subject to the censorship law, though foreigners rarely submit material to the censor and enforcement is less strict. However, some foreigners have run afoul of the authorities.

In late June, the British Broadcasting Corporation aired a documentary examining the Israeli nuclear establishment's history, Vanunu's imprisonment and illnesses among former workers at the Dimona reactor.

The Israeli government retaliated within days. It stopped providing spokesmen for BBC stories and prohibited its reporters from attending government press conferences. "They are trying to demonize the state of Israel," Seaman, the head of the press office, said of BBC in an interview in August. "We are not cooperating with them."

Tim English, a BBC spokesman, said the broadcaster stands by the accuracy and fairness of its program.

Censorship extends to academics, too. Cohen, the Israeli-American scholar, has written a second book that criticizes Israel's nuclear secrecy as 'anacronistic.' In July, his Israeli publisher submitted the manuscript to the censor in hopes of publishing it in Hebrew. Cohen said a decision is expected soon, but that early signs have been that the censor is taking a tough line.

"This will show how far the Israeli government is willing to go to allow serious discussion of the issue," Cohen said in an interview. Israel's parliament was dragged into the nuclear debate briefly on Feb. 2, 2000. Issam Makhoul, one of 10 Arab-Israelis in parliament, got the item on the agenda by petitioning the Supreme Court after being rebuffed seven times.

"The entire world knows that Israel is a huge warehouse of nuclear, biological and chemical weapons that serves as a cornerstone of the nuclear arms race in the Middle East," said Makhoul, whose speech was protected by parliamentary immunity.

Several members of parliament walked out on the 10-minute speech. Others responded with angry shouts. "This is putting lives in danger," said one member, Moshe Gafni. Chaim Ramon, a cabinet minister who spoke on behalf of the government, said no democratic country invites its enemies to listen in on discussions of nuclear arms policy. "Do you want us to announce to Iran and Iraq exactly what we have?" he asked.

Sitting in his cluttered office in Haifa recently, Makhoul defended his attempt to spark a debate and argued that the issue is more pressing now.

"The American administration decided to destroy weapons of mass destruction in Iraq and they are threatening Iran," he said. "They cannot continue giving a blind eye to what is going on in Israel."

Some experts contend Israel no longer needs nuclear weapons because Iraq is no longer a threat and Israel's conventional forces are superior to any combination of Arab armies. Israel's problems with Palestinian extremists, they argue, cannot be remedied by nuclear strikes.

"Israel has a direct interest in making sure no Muslim state acquires the one weapon that could offset its conventional superiority, a nuclear bomb," said Cirincione, the nonproliferation director at Carnegie in Washington. "One way to do that is by putting its own nuclear weapons on the table."

Some Arab leaders advocate declaring the Middle East a zone free of weapons of mass destruction. The process would be long, starting with mutual pledges to give up weapons and creation of a regime to verify compliance.

Few Israelis think this is the right time to discuss it again because of the level of violence with Palestinians.

"Israel could accept the idea after two years of comprehensive peace in the Middle East," said Ephraim Kam, deputy director of the Jaffa Center for Strategic Studies in Tel Aviv."Only then could we consider changing our nuclear position."