The Obama administration does not know Israel's intentions regarding potential military action against Iran, and the uncertainty is stoking concern in Washington, where the preferred course for now is sanctions and diplomatic pressure.

Although Israel remains one of the United States' closest allies and the two countries' officials are in regular contact, U.S. officials have a sense of opacity regarding what might prompt an Israeli military strike on Iranian nuclear sites, and about when such an attack might occur, a senior U.S. national security official told Reuters.

Two key U.S. senators acknowledged on Tuesday that there are gaps in U.S. knowledge about Israeli leaders' thinking and intentions.

I don't think the administration knows what Israel is going to do. I'm not sure Israel knows what Israel is going to do ... That's why they want to keep the other guys guessing. Keep the bad guys guessing, said Sen. Carl Levin, D-Mich., chairman of the Senate Armed Services Committee.

Sen. John McCain of Arizona, the senior Republican on the committee, echoed Levin's view: I'm sure (administration officials) don't know what the Israelis are going to do. They didn't know when the Israelis hit the reactor in Syria. But the Israelis usually know what we're going to do.

In one way, the ambiguity is an advantage for the United States, because Washington could claim it had no foreknowledge of any Israeli attack, which would almost certainly increase anti-American sentiment in the Middle East.

Israeli leaders have not suggested an attack on Iran's suspected nuclear weapons sites is imminent. But neither have they - or U.S. President Barack Obama, for that matter - ruled it out.

Israel, widely believed to have the only nuclear arsenal in the Middle East, says a nuclear-armed Iran would threaten its existence. Iran says its nuclear program is entirely peaceful and denies Western accusations it is seeking an atomic bomb.

'UNINTENDED CONSEQUENCES'

The uncertainty comes amid extraordinarily sharp public warnings in the last few weeks by U.S. Defense Secretary Leon Panetta about the potential unintended consequences of military action against Iran.

Panetta told a forum in Washington last week that an attack on Iran would risk an escalation that could consume the Middle East in confrontation and conflict that we would regret.

It could disrupt the fragile economies of the United States and Europe, spark a popular backlash in Iran bolstering its rulers and put U.S. forces in the region in the firing line, he said. The United States would obviously be blamed and we could possibly be the target of retaliation from Iran, striking our ships, striking our military bases, Panetta said.

Gen. Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, told Reuters in an interview he did not know whether the Jewish state would give the United States notice ahead of time if it decided to act.

An Israeli government official said, Israel and the United States are in close and continuous communication on the threat posed to world security by the Iranian nuclear program. We appreciate President Obama's determination to prevent Iran from developing a nuclear weapon. The official declined to comment further.

At the same time, however, Obama's relations with Israeli leaders have not been particularly warm. He has not visited the country as president.

A former U.S. government official said: There are plenty of instances when the Israelis have undertaken action without informing the United States first. So not always should we assume a level of coordination in advance on all issues.

REPEAT PERFORMANCE?

Bruce Riedel, a former senior CIA expert on the Middle East who has advised Obama, said, Israel has a long history of conducting military operations from Baghdad to Tunis without giving Washington advance notice.

Riedel said the White House wants to send Israel a strong message that the United States does not expect to be blindsided by its ally. Obama wants Bibi to understand unequivocally he does not want a repeat performance in Iran, he said, referring to Israeli Prime Minister Benjamin Netanyahu by his nickname.

The Obama administration suspects that Israeli leaders have marked out for themselves certain red lines related to Iranian nuclear progress which could trigger Israeli military action if they are crossed, one U.S. official said, speaking on condition of anonymity.

But Obama administration policymakers are plagued by a sense of opacity in their understanding of where the Israeli red lines are drawn, the official added.

Two other U.S. officials, also speaking on condition they not be named, said Washington is deeply concerned Israel, unconvinced sanctions and diplomatic pressure will halt Iran's nuclear program, could eventually decide to take action on its own.

By the same token, one of the U.S. officials said, speeches and statements by Israeli leaders, like an address by Netanyahu on Sunday in which he talked about making the right decision at the right moment even if allies object, could be politically motivated.

Under this interpretation, Netanyahu and other Israeli officials may be playing to domestic audiences or trying to put pressure on the international community to do more on Iran.

(Additional reporting by Susan Cornwell and Andrew Quinn; editing by Mohammad Zargham)

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