The United States and Israel won't discuss it or even admit it exists. But experts say Israel has had nuclear weapons since the late 1950s.
When Israeli Prime Minister Benjamin Netanyahu drew a red line across a cartoon image of an Iranian nuclear bomb at the United Nations last week, his goal was to prod the United States and other countries to take possible military action against what he sees as an imminent threat. Left unsaid is the fact that Israel possesses the Middle East's biggest nuclear weapons program, even though it denies its very existence.
"It's one of the best-known secrets," said Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists. "We know that Iran does not have nuclear weapons. We know that Israel has nuclear weapons."
Netanhayu warned that Iran would be close to having enough weapons-grade nuclear material by next spring or summer. President Barack Obama has said he wants more time for economic sanctions to take effect, and for diplomacy to work.
Neighbors Syria, Iraq and Iran are signatories to the nuclear non-proliferation treaty, but Israel is not. The only other nuclear nations that haven't signed on are Pakistan, India and North Korea. That means inspectors from the International Atomic Energy Agency (IAEA) are not allowed to visit Israel to determine whether nuclear material is entering or exiting the country, or any other aspects of weapons development. The only exception is for a small research reactor.
IAEA officials in Vienna refused to comment about any aspect of Israel's nuclear weapons program.
But according to Kristensen, the U.S. intelligence community estimates that Israel has 80 to 90 nuclear warheads. Israel has the capability of launching these warheads via ballistic missiles (called the Jericho 2) that have a range of about 1,500 kilometers, or 930 miles, on missiles on U.S. supplied F-15s and F-16 fighter jets and on Cruise missiles aboard diesel-powered submarines supplied by Germany.
Why the secrecy? Kristensen and other experts say the program has been kept quiet since Israel began it in the late 1950s. Documents from the Nixon Administration declassified in 2006 revealed that President Richard Nixon agreed to allow Israel's leader Golda Meier to continue developing nuclear weapons as long as Israel didn't acknowledge it or conduct public weapons tests, according to Avner Cohen, senior fellow at the Monterey Institute for International Studies and author of several books on Israel's nuclear program.
"That was the fundamental presumption of the program and nobody from the program has ever talked in public," Cohen said. "The program has been wrapped with a great deal of secrecy."
An Israeli technician who worked at the Israeli Dimona nuclear complex, Mordechai Vanunu, revealed some details about Israel's nukes to the British press back in 1986. He was then lured to Italy, kidnapped by Israeli agents, and spent the next 18 years in prison, much of it in solitary confinement.
"It's the most sensitive issue in Israel," Cohen said. "Very few people have any reliable information and information is very scarce."
Recent history demonstrates that Israel hasn't hesitated to use force to stop its neighbors from joining the nuclear weapons club. In 1981, Israeli jets destroyed an Iraqi weapons facility and in 2007 Israeli fighters bombed a Syrian facility. The IAEA later said the facility was likely a nuclear reactor.
At some point Israel could change its sealed-lips policy, according to Kristensen. But it probably won't happen until the conflict with Iran is resolved.
"Everyone suspects that if Israel went out and publicly declared we will have (a nuclear weapon), then Iran would jump on it and say that would give them legitimacy to develop their own," Kristensen said. "They have to be careful what they say right now. The only way to solve the issues is if you manage to get all countries in the region to talk about it. But it's an illusion that you could solve the bigger issue unless Israel is willing to talk about it as well."