More than 100 Israeli F16 and F15 jets were involved in the exercise
Israel has carried out an exercise that appears to have been a rehearsal for an attack on Iran's nuclear facilities, US officials have told the New York Times.
More than 100 Israeli fighter jets took part in manoeuvres over the eastern Mediterranean and over Greece in the first week of June, US officials said.
Iran insists its programme is peaceful, but Israel sees Iran's development of the technology as a serious threat.
Tehran is defying a UN demand that it stops the enrichment of uranium.
The UN Security Council approved a third round of sanctions against Iran over the issue in March 2008.
The Israeli exercise, it seems, was designed to send a message to Tehran that Israel has the power and will to attack if it thought Iran was close to getting a nuclear weapon, the BBC's Jeremy Bowen reports.
None of what has been said and done so far means an attack on Iran is coming and talk of one faded out after US intelligence reported at the end of 2007 that Iran had given up its nuclear weapons programme, he notes.
But now it is back and that is significant, our Middle East Editor says.
Mohammed ElBaradei, head of the UN's nuclear watchdog, said an attack would put Iran on a "crash course" to building nuclear weapons.
"A military strike, in my opinion, would be worse than anything possible - it would turn the region into a fireball," he told Al Arabiya television in an interview.
"It would make me unable to continue my work," he said.
Several US officials briefing the New York Times said the exercise was intended to demonstrate the seriousness of Israel's concern over Iran's nuclear activities, and its willingness to act unilaterally.
"They wanted us to know, they wanted the Europeans to know, and they wanted the Iranians to know," a Pentagon official is quoted as saying by the newspaper.
"There's a lot of signalling going on at different levels."
The exercise involved Israeli helicopters that could be used to rescue downed pilots, the newspaper reported.
The helicopters and refuelling tankers flew more than 1,400km (870 miles), roughly the distance between Israel and Iran's main uranium enrichment plant at Natanz.
The New York Times reported that Israeli officials declined to discuss the details of the exercise.
A spokesman for the Israeli military said the air force "regularly trains for various missions in order to confront and meet the challenges posed by the threats facing Israel".
The US state department would not comment on the Israeli exercise.
A spokesperson said the US was focused on making diplomacy work with Iran but insisted that all options were still on the table, echoing remarks made recently by President George W Bush.
Diplomats in Washington described the exercise as muscle-flexing, a message that Israel would be ready to take unilateral action against Iran if needed.
But they add that if Israel ever decides to strike, there will be little advance warning - just like when it targeted a suspected nuclear reactor in Syria last September and Iraq's nuclear plant in Osirak in 1981.
Our Middle East editor adds that, at the UN, Russia and others believe attacking Iran would only make matters worse, and talking about it undercuts the diplomacy.
Israeli Prime Minister Ehud Olmert warned on 4 June that drastic measures were needed to stop Iran obtaining nuclear weapons.
He said Iran must be shown there will be devastating consequences if it did develop such weapons.
Israeli deputy Prime Minister Shaul Mofaz - a former defence minister - said earlier this month that military strikes to stop Iran developing nuclear weapons looked "unavoidable".
In 1981, Israeli jets bombed the Iraqi nuclear reactor at Osirak, 30km (18 miles) outside Baghdad.
Israel said it believed the French-built plant was designed to make nuclear weapons that could be used against Israel.