* Stark U.S. criticism surprises Israeli leadership
* Israel seeks explicit Washington warning for Iran
* Indications U.S. president might toughen language
By Crispian Balmer
JERUSALEM, Sept 4 (Reuters) - Stunned by a rebuke from the United States' top general, Israel is preparing a climbdown strategy in its war of words over Iran's nuclear programme, aware that its room for manoeuvre is shrinking rapidly.
Anxious to prevent any flare-up in the Middle East ahead of November elections, there is also a good chance that U.S. President Barak Obama will provide Israel with enough cover to avoid a loss of face, analysts say.
A burst of bellicose rhetoric over the last month led Western allies to fear that Israel was poised to launch a unilateral strike against Iran in an effort to hobble the Islamic Republic's contested nuclear facilities.
Convinced Iran is seeking the atomic bomb, Israeli leaders have warned of a possible Holocaust if Tehran is not stopped; but the sabre-rattling clearly riled Washington, while failing to rally domestic public opinion behind a perilous war.
In a move that dismayed Israeli ministers, U.S. Chairman of the Joint Chiefs of Staff, General Martin Dempsey, told reporters in Britain last week that the United States did not want to be "complicit" in an Israeli attack on Iran.
He also warned that go-it-alone military action risked unraveling an international coalition that has applied progressively stiff sanctions on Iran, which insists that its ambitious nuclear project is purely peaceful.
Dempsey's stark comments made clear to the world that Israeli Prime Minister Benjamin Netanyahu was isolated and that if he opted for war, he would jeopardise all-important ties with the Jewish state's closest ally.
"Israeli leaders cannot do anything in the face of a very explicit 'no' from the U.S. president. So they are exploring what space they have left to operate," said Giora Eiland, who served as national security adviser from 2003 to 2006.
"Dempsey's announcement changed something. Before, Netanyahu said the United States might not like (an attack), but they will accept it the day after. However, such a public, bold statement meant the situation had to be reassessed."
Pointing to a possible way out, Netanyahu has since said that more explicit international warnings could prevent war, indicating he wanted the United States to provide Tehran with unambiguous options to halt its nuclear activity or face war.
"The greater the resolve and the clearer the red line, the less likely we'll have conflict," he said on Monday.
Positions are likely to be clarified at an expected meeting late this month between U.S. President Barack Obama and Netanyahu when the Israeli leader addresses the U.N. General Assembly in New York.
"That will be a crucial encounter. They will have to reach an understanding there. At the end of the day, you do reach an understanding, always," said Eiland, who had numerous dealings with Washington during his time as national security adviser.
There are already signs that Obama is prepared to raise the pressure on Iran.
On Monday, his Democratic Party released its election platform, saying the window for diplomacy would not remain open "indefinitely" and explicitly raised the threat of "military force" if Iran did not "live up to its obligations".
The programme appeared to be more toughly worded than public declarations from Obama, but it is not politically binding. An official within the prime minister's office said Israel wanted to hear cast iron commitments from Obama's own mouth.
"We want to hear a concrete declaration from the president, not vague promises that he will guarantee Israeli security," the official said, declining to be named.
The official noted the tough stance the Americans took in 2011, warning they would not tolerate any move by Iran to carry through with a threat to close the Strait of Hormuz, and hoped to see similar clarity applied to the nuclear programme.
Netanyahu met the heads of Israel's intelligence community on Tuesday for an annual briefing to the security cabinet, where they were expected to present their latest assessments on Iran and the situation in southern Lebanon, amongst other things.
The leader of Lebanon's Iranian-backed militant group Hezbollah said on Monday Iran could hit U.S. bases in the Middle East in response to any Israeli attack on its nuclear sites.
"A decision has been taken to respond and the response will be very great," Hezbollah Secretary-General Sayyed Hassan Nasrallah told a local television station, ramping up the rhetoric which has raged through the region this summer.
Israel's vice prime minister, Moshe Yaalon, issued his own bleak warning to Hezbollah to stay out of any possible conflict.
Talking to 100fm radio on Friday, he said Hezbollah had some 60,000 missiles and rockets, but that Israel had a much bigger arsenal. "Therefore, they need to understand that if they fire rockets and missiles, Hezbollah will pay a heavy price and the state of Lebanon will pay a heavy price until they stop."
Behind all the bluff and bombast, there is no question that the military in Israel is reviewing all its plans in case of conflict. Three officials told Reuters preparations for a possible, imminent, unilateral strike on Iran were "serious".
Civilians are also being readied for possible bloodshed, with the military issuing a booklet last week on how to deal with possible emergency, and lines forming at distribution centres across the country for free gas masks.
Despite all the obvious activity, it is hard to shake off a sense of scepticism. Although Israel is believed to have the region's only nuclear arsenal, it lacks the sort of conventional firepower pundits believe is necessary to put a serious dent in Iran's far-flung, well-defended atomic installations.
"All this talk of war is bullshit. If they could do it, then they would have already done it long ago," a senior European diplomat in Israel said. (Additional reporting by Dan Williams and Jeffrey Heller; editing by Ralph Boulton)