Several US officials said the exercise appeared to be an effort to develop the military's capacity to conduct long-range strikes and to demonstrate the seriousness with which Israel views Iran's nuclear programme.

More than 100 Israeli F-16 and F-15 fighters participated in the manoeuvres, which were carried out over the eastern Mediterranean and over Greece during the first week of this month, US officials said.

The exercise also included Israeli helicopters that could be used to rescue downed pilots. The helicopters flew more than 1,450km, which is about the same distance between Israel and Iran's uranium enrichment plant at Natanz, US officials said.

The Israeli military declined to discuss the details of the exercise. A spokesman would say only that the country's air force 'regularly trains for various missions in order to confront and meet the challenges posed by the threats facing Israel'.

But the scope of the Israeli exercise virtually guaranteed that it would be noticed by US and other foreign intelligence agencies.

A senior Pentagon official who had been briefed on the exercise said the it appeared to serve several purposes.

One Israeli goal, the official said, was to practise flight tactics, aerial refuelling and other details of a possible strike against Iran's nuclear installations and its long-range conventional missiles.

A second was to send a clear message to the United States and other countries that Israel was prepared to act militarily if diplomatic efforts to stop Iran from producing bomb-grade uranium continued to falter.

'They wanted us to know, they wanted the Europeans to know and they wanted the Iranians to know,' the Pentagon official said.

Several US officials said they did not believe the Israeli government had concluded that it must attack Iran, and did not think that such a strike was imminent.

In a recent interview, Mr Shaul Mofaz, a former Israeli defence minister who is now a deputy prime minister, had warned that Israel might have no choice but to attack. 'If Iran continues with its programme for developing nuclear weapons, we will attack,' Israeli newspaper Yediot Aharonot quoted him as saying on June 6, the day after the unpublicised exercise ended.

But Mr Mofaz was criticised by other Israeli politicians as seeking to enhance his own standing as questions mounted about whether embattled Israeli Prime Minister Ehud Olmert could hang on to power.

Israeli officials told their US counterparts that Mr Mofaz's statement did not represent official policy. But they also said that Israel had prepared plans for striking nuclear targets in Iran, and could carry them out if needed.

Iran has shown signs that it is taking the Israeli warnings seriously by beefing up its air defences in recent weeks, including increasing air patrols.

It is also taking steps to better defend its nuclear facilities. Two sets of advance Russian-made radar systems were recently delivered to Iran, which will enhance the country's ability to detect planes flying at low altitude.

For both the US and Israel, Iran's nuclear programme has been a persistent worry. A National Intelligence Estimate that was issued last December by US intelligence agencies asserted that Iran had suspended work on weapons design in late 2003.

It also noted that Iran's work on uranium enrichment and on missiles, two steps that Iran would need to take to field a nuclear weapon, had continued.

Last month, the International Atomic Energy Agency reported that Iran's suspected work on nuclear matters was a 'matter of serious concern', and that the Iranians owed the agency 'substantial explanations'.

NEW YORK TIMES, REUTERS

Past strikes

OVER the past 30 years, Israel has carried out unilateral strikes against possible nuclear sites in the Middle East: