Israeli officials believe that destroying all three sites would delay Iran's nuclear programme indefinitely and prevent them from having to live in fear of a "second Holocaust".

The Israeli government has warned repeatedly that it will never allow nuclear weapons to be made in Iran, whose president, Mahmoud Ahmadinejad, has declared that "Israel must be wiped off the map".
 
Robert Gates, the new US defence secretary, has described military action against Iran as a "last resort", leading Israeli officials to conclude that it will be left to them to strike. Israeli pilots have flown to Gibraltar in recent weeks to train for the 2,000-mile round trip to the Iranian targets. Three possible routes have been mapped out, including one over Turkey.

Air force squadrons based at Hatzerim in the Negev desert and Tel Nof, south of Tel Aviv, have trained to use Israel's tactical nuclear weapons on the mission. The preparations have been overseen by Major General Eliezer Shkedi, commander of the Israeli air force.

Sources close to the Pentagon said the United States was highly unlikely to give approval for tactical nuclear weapons to be used. One source said Israel would have to seek approval "after the event", as it did when it crippled Iraq's nuclear reactor at Osirak with airstrikes in 1981.

Scientists have calculated that although contamination from the bunker-busters could be limited, tons of radioactive uranium compounds would be released.

The Israelis believe that Iran's retaliation would be constrained by fear of a second strike if it were to launch its Shehab-3 ballistic missiles at Israel.

However, American experts warned of repercussions, including widespread protests that could destabilise parts of the Islamic world friendly to the West.

Colonel Sam Gardiner, a Pentagon adviser, said Iran could try to close the Strait of Hormuz, the route for 20% of the world's oil.

Some sources in Washington said they doubted if Israel would have the nerve to attack Iran. However, Dr Ephraim Sneh, the deputy Israeli defence minister, said last month: "The time is approaching when Israel and the international community will have to decide whether to take military action against Iran."

Citing unidentified Israeli military sources, The Sunday Times said the proposals drawn up in Israel involved using so-called "bunker-buster" nuclear weapons to attack nuclear facilities at three sites south of the Iranian capital.

Israel has never confirmed it has nuclear weapons, although the Jewish state is widely believed to possess a significant stockpile.

Iran says its nuclear program is solely for peaceful purposes like generating electricity.

The Sunday Times reported that Israeli military officials believed Iran could produce enough enriched uranium to build nuclear weapons within two years, and the newspaper said Israeli pilots had made flights to the British colony of Gibraltar to train for the 2,000-mile round trip to the Iranian targets.

Israeli Prime Minister Ehud Olmert's office declined to comment on the report. "We don't respond to publications in The Sunday Times," said spokeswoman Miri Eisin.

However Foreign Ministry spokesman Mark Regev denied the report, saying: "if diplomacy succeeds, the problem can be solved peaceably."

The United States and its allies suspect Tehran of secretly trying to produce atomic weapons there.

Some view Israeli officials' occasional implied threats as a means of pressuring the world community to take action, building on the recent United Nations Security Council decision to impose some economic sanctions on Tehran for its refusal to suspend uranium enrichment.

Some analysts viewed Sunday's report as another element of delicate diplomacy.

"I refuse to believe that anyone here would consider using nuclear weapons against Iran," Reuven Pedatzur, a prominent defense analyst and columnist for the Israeli daily Haaretz, told the AP. "It is possible that this was a leak done on purpose, as deterrence, to say: 'Someone better hold us back, before we do something crazy.'"

Ephraim Kam — a former senior intelligence official now at Tel Aviv University's Institute for National Strategic Studies — also suggested the report should not be taken literally. "No reliable source would ever speak about this, certainly not to the Sunday Times," he said.