Israel has acquired three diesel submarines that it is arming with newly designed cruise missiles capable of carrying nuclear warheads, according to former Pentagon and State Department officials, potentially giving Israel a triad of land-, sea- and air-based nuclear weapons for the first time.

The U.S. Navy monitored Israeli testing of a new cruise missile from a submarine two years ago off Sri Lanka in the Indian Ocean, according to former Pentagon officials.

One former senior American official said U.S. analysts have studied the nuclear capability of the cruise missile. But, according to a former Pentagon official, "It is above top secret knowing whether the sub-launched cruise missiles are nuclear-armed." Another former official added, "We often don't ask."

The possible move to arm submarines with nuclear weapons suggests that the Israeli government might be increasingly concerned about efforts by Iraq and Iran to develop more accurate long-range missiles capable of knocking out Israel's existing nuclear arsenal, which is primarily land-based.

Although developing a sea-based leg would preserve the deterrent value of Israel's nuclear force, according to analysts, it would complicate U.S. efforts to keep other countries in the Middle East and elsewhere from seeking to acquire nuclear arms. It also could spur a nuclear arms race in the Middle East.

Israel has long refused to confirm or deny it has nuclear weapons. U.S. analysts say it has a modest arsenal of short- and medium-range nuclear-capable missiles, nuclear bombs that could be delivered from jet fighters and Harpoon missiles that could be launched from planes or ships.

Mark Regev, spokesman for the Israeli Embassy, confirmed that his country had recently acquired three submarines from Germany but would not comment on whether they were being outfitted with nuclear weapons. "There has been no change in Israel's long-standing position not to introduce nuclear weapons in the Middle East," Regev said.

A book published this week by the Carnegie Endowment for International Peace reported that Israel was attempting to arm its diesel submarines with nuclear cruise missiles.

"Probably the most important nuclear-related development in Israel is the formation of its sea-based nuclear arm," wrote Joseph Cirincione, director of the Carnegie Endowment's nonproliferation project and a former staff member of the House Armed Services Committee who served as chief author of the book.

The U.S. government "favors" Israel's preserving the ambiguity surrounding its nuclear force, just as it has since the late 1960s, a former senior U.S. diplomat said. "It gives it a strategic deterrence," he said, adding, "If [Israel] were being explicit, that would create problems with its neighbors like Egypt and Syria . . . whose leaders years ago agreed that [ambiguity] did not pose an offensive threat to them."

Iraq and Iran, he added, are different because "they are destabilizing" countries and could launch a first strike against Israel or U.S. forces in the region if they succeed in developing and deploying nuclear weapons.

There have been published reports going back to 1998 that describe Israel's acquisition of the diesel submarines and its testing of a cruise missile.

In an article two years ago in the Israeli newspaper Ha'aretz, Reuven Pedatzur, a former Israeli fighter pilot and director of the Galili Center for Strategy and National Security, wrote that Israel was motivated by "the need to find deterrence solutions . . . from the probability that during the next decade Iran, and maybe even Iraq, will acquire the nuclear ballistic capability to hit Israeli targets."

Pedatzur said that faced with that threat, a submarine force armed with missiles is a reliable deterrent because Israel's enemies would not be able to locate and destroy them and thus "that it is impossible to avoid their lethal counterstrike."

The Carnegie Endowment book said Israel "is believed to have deployed" 100 Jericho short-range and medium-range missiles that are nuclear-capable. In addition, it has nuclear bombs that could be delivered from U.S.-made F-16 jet fighters and U.S.-built Harpoon missiles that could be launched from planes or ships.

Israel's nuclear-capable, sea-launched cruise missiles were tested in May 2000, the book said, and might have a range of more than 900 miles. With three submarines, Israel could "have a deployment at sea of one nuclear-armed submarine at all times," the book said.

"Such a survivable deterrent is perceived as essential because of Israel's unique geopolitical and demographical vulnerability to nuclear attack, and one that no potential enemy of Israel could ignore," it said.

Cirincione said Israel's possession of nuclear weapons and modernization of its systems creates an "extremely difficult situation" not just for the United States, but also for preventing other countries that have signed the Nuclear Non-Proliferation Treaty from breaking away. Israel's possession of weapons remains officially ambiguous, but Israel, along with Pakistan and India, did not sign the treaty.

Israel is only one of 15 countries discussed in the book, which describes the spread of nuclear, chemical and biological weapons and their missile delivery systems. It updates a similar volume produced by the Carnegie Endowment four years ago.

Cirincione said at least eight countries have nuclear weapons -- the United States, Russia, Britain, France, China, Israel, India and Pakistan -- and three more are apparently seeking them -- Iraq, Iran and North Korea. Four countries, he said, have in recent years given up their weapons -- South Africa and the former Soviet republics Ukraine, Belarus and Kazakhstan.

The book attributed Iran's decision to seek nuclear, chemical and biological weapons to its experience during its war with Iraq in the 1980s, when Iraqi President Saddam Hussein used chemical weapons against Iranian forces. Iran is influenced by its "extended neighborhood [where] it sees Israel, India and Pakistan with advanced nuclear weapons" and Iraq's weapons program no longer subject to inspection by the United Nations, the book said.

The authors said U.S. sanctions against Iran, which have hurt its ability to build conventional military forces, "have likely worked toward reaffirming belief in the utility of unconventional weapons."

Iraq's search for nuclear and biological weapons rests on Hussein's desire to be the "dominant power in the Middle East" and his belief that "a nuclear bomb would provide him with the ultimate symbol of military power," the book said. It said "Iraq may have a workable design for a nuclear weapon" and that if Baghdad "were to acquire material from another country, it is possible that it could assemble a nuclear weapon in months."