The Obama administration is increasingly anxious about Israeli leaders' provocative public comments on Iran's nuclear program but does not have hard proof that it will strike Iran in the next few months, U.S. and European officials said.

The U.S. uncertainty and lack of information about Israel's plans on Iran were behind an alarming assessment of the situation reportedly voiced by Defense Secretary Leon Panetta, the officials said.

Open gallery view Iranian students stand in front of former U.S. embassy in Tehran, November 4, 2011. Credit: Reuters

David Ignatius, a Washington Post columnist who specializes in intelligence matters, reported that Panetta believed there was a "strong likelihood" that Israel would attack Iran's nuclear program within the next six months - as early as April, Ignatius wrote.

Three U.S. officials who follow the issue said their understanding was that the United States did not have concrete intelligence suggesting an attack by Israel on Iran in that time frame was likely or actively being prepared.

The current U.S. assessment is that for months Israel had been making contingency plans and tentative preparations both for such an operation and for possible Iranian retaliation, two of the officials said.

Nonetheless, said the officials, indications were that Israel's leadership had not made a final decision to attack Iran.

Ken Pollack, a former White House and CIA official with expertise on the Gulf, said the sudden rise in public discussion of an Israeli strike on Iran's known nuclear sites - including increasingly dire warnings from Israel's leaders - were misleading.

"If Israel has a good military option, they just take it, they don't talk about it, they don't give warnings," said Pollack, director of the Saban Center for Middle East Policy at the Brookings Institution. "So the fact that they are talking about it, to me, is one tip-off that they don't have a good military option.

"We should never rule out the possibility of an Israeli strike and the odds have probably increased in recent months as a result of a number of different factors. But ... there are a lot of disincentives that have prevented Israel from launching a strike for 10 years," Pollack said.

Panetta was vague when asked by journalists to confirm what the Washington Post had reported. "Frankly, I'm not going to comment on that," he told reporters travelling with him in Europe. "David Ignatius, you know, can write what he will but, you know, with regards to what I think and what I view, I consider that to be an area that belongs to me and nobody else."

When pressed further, Panetta said: "There really isn't that much to add except that, you know, that they're considering this and, you know, we have indicated our concerns." Asked about the background to Panetta's reported views, one of the U.S. officials noted that Israeli leaders, including Prime Minister Benjamin Netanyahu and Defense Minister Ehud Barak, had been "increasingly vocal" in expressing concern that Israel might be "running out of time" to stop Iran from building a nuclear bomb. The official said that some Israelis have indicated their view that in the next three or four months the need for Israeli action could become critical.

But the view of many career experts inside the U.S. government is that Iran's nuclear development program, which Tehran insists is for civilian nuclear purposes, is unlikely to pass the point of no return in that time.

Earlier this week, U.S. Director of National Intelligence James Clapper testified before Congress and publicly re-stated the long-standing view of U.S. intelligence agencies that Iran's leaders have not yet decided to build a nuclear weapon.

Many, if not most, Western experts believe it would take Iran at least a year to build a weapon once leaders decided to go ahead.

But some Israel leaders and experts believe that an attack would have to be launched earlier if Iran's nuclear effort is to be set back seriously. Barak has warned that Iran's nuclear research could soon pass into what he called a "zone of immunity," protected from outside disruption.

Barak recently was quoted telling a security conference in Israel, "Later is too late," one of the U.S. officials noted. The official said that U.S. policymakers had to be concerned about the possibility of an early Israeli attack "given that Barak and Netanyahu seem so determined to do it."

In January an Iranian nuclear scientist was killed by man who attached a bomb to his car - the fifth such attack in two years. Israel's military chief said Iran could expect more such incidents.

One of the U.S. officials said that while Israel may have the military capability to delay Iran's nuclear effort for a period of time, to deal the Iranian program a serious and long-term setback would require additional military power, presumably from the United States.

But Panetta's alleged remarks and other Obama administration's statements indicate the White House is focused on dissuading Israel from taking action - and distancing itself from an Israel strike if persuasion fails.

A strike on Iran and Iran's response, including attempts to close the Strait of Hormuz, which is vital for oil shipments, could seriously harm the U.S. economy, jeopardizing President Barack Obama's chances for re-election. Obama also would likely come under intense domestic pressure to back Israel's actions.

"The U.S. is not too excited about engaging with Israel or being part of anything at this point," one official said.

A European defense analyst, who has access to classified all-source intelligence, said that while Iran's behavior was relatively predictable, the greatest uncertainties facing the U.S and its allies stemmed from Israel's stance.

Despite internal power squabbles, the analyst said, Iran has been "quite restrained and limited in its responses." Recent inflammatory comments by Iranian leaders, such as threats to block the Strait of Hormuz, were relatively low-intensity compared to other threats and physical confrontations in the Gulf of past years.

"Israel is, practically speaking, the wild card in the pack," the analyst said. "We have no specific information on when or if they will attack but based on their past history and current stance, it is something we do expect at some point."