The U.S. State Department on Tuesday criticized reported comments by a senior U.S. defense official who said there was an increasing likelihood Israel would attack Iran over its nuclear program, calling his statements "foolish."

The unidentified defense official told ABC News that it was increasingly likely Israel would attack Iran, and that Washington was concerned Iran would strike both the United States and Israel in retaliation.

State Department spokesman Tom Casey said in response to the report: "I have no information that would substantiate that, and I think it's rather foolish of people who often have no clue what they're talking about to assert things and not even have the courtesy to do so on the basis of their name."

Meanwhile Tuesday, an Israeli official said that Prime Minister Ehud Olmert secretly visited the site of Israel's main nuclear reactor, against the backdrop of speculation that Israel may soon attack Iran's nuclear facilities.

The official would not say what the purpose of Olmert's visit was.

Earlier Tuesday, ABC News quoted a U.S. defense official as saying that two "red lines" would prompt Israel to strike Iran. The first trigger would be once enough highly enriched uranium is produced at Iran's Natanz nuclear facility to create a nuclear bomb, which U.S. and Israeli assessments predict to occur will the end of this year or next.

"The red line is not when they get to that point, but before they get to that point," the official was quoted as saying. "We are in the window of vulnerability."

The second trigger, according to the official, would be linked to Iran's acquisition of the SA-20 air defense system it is purchasing from Russia.

The official said Israel may be likely to attack Iran before the system is put into place and Tehran's deterrence bolstered.

Washington is also concerned that Israel may carry out an attack before the next U.S. president is sworn in, according to the report.

The ABC News report was just the most recent in a series of media revelations on the possibility that Israel would use force to stop the Iranian nuclear program.

Previous reports included Transportation Minister Shaul Mofaz's comments on the matter, reported in the New York Times and never denied by Israel, on a large military exercise the Israeli Air Force conducted last month in preparation for such an attack.

There was even a day filled with rumors and false reports of an actual Israeli attack, which drove down world markets and pushed up oil prices.

In the middle of all this, Iran revealed it had sentenced to death one of its citizens accused of being an Israeli spy for collecting information on Iran's nuclear plans.

A senior American defense official called the IAF military exercise "not a rehearsal, but basic, fundamental training" required for operation against Iran, ABC reported.

"The Israeli air force has already conducted the basic exercise necessary to tell their senior leadership, 'We have the fundamentals down.' Might they need some more training and rehearsals? Yes. But have they done the fundamentals? I think that is what we saw," the official told ABC News.

The increasing media interest all over the globe in a possible Israeli attack can be explained by the convergence of a number of issues coming to a head in a short period.

In addition, to Israeli reports of Iranian nuclear progress, the U.S. election campaign will soon enter its final phase, and even if the next administration - whoever its leader may be - is no less supportive of Israel than the present Bush administration, it is clear that its first year will be dedicated more to learning about complex international issues than to actually dealing with them.

In the background, the effects are still being felt of the American intelligence community's report from last December stating that Iran had suspended its military nuclear weapons program. President George W. Bush, in his recent talks with Israel, expressed serious doubts about the conclusions.

But the common opinion among Israeli politicians and the defense establishment is that Bush will find it difficult to initiate an American attack on Iran in light of the wide-scale opposition in the public and the Congress, as well as in parts of the U.S. defense establishment.

This leaves the possibility of an attack on Iran solely in Israel's hands. As far as is known, no decisions have yet been made, but the direction is much clearer than in previous years. It is worth remembering Ehud Olmert's comments to Bush at the end of their meeting in the White House a month ago. He said that every day that passes "brings us closer to the solution" to the Iranian nuclear problem.

What Bush and Olmert tell each in in private no one else knows, but it certainly sounds like a hint to their discussion of a "green light" on the military option.

The IAF exercise, the second in half a year, should also be viewed as an attempt to improve the IDF's preparedness for such an attack. Israel is broadcasting that all its options are still open, and is also informing the U.S. that it is serious. Such firmness might also help advance U.S. willingness to support such an attack, as Israel would need quite a bit of assistance from the Americans if it decides to attack: from coordination of flight paths - due to the U.S. presence in Iraq - along with intelligence, and also bombs that could penetrate the Iranian facilities deep underground.

Last week the U.S. chairman of the Joint Chiefs of Staff, Mike Mullen, visited Israel and met with the heads of the Israeli defense establishment.

The key question relates to the ability of Israel's leadership to make decisions in the present political - and criminal - situation. From what has been allowed to be reported about last year's attack on the Syrian installations, it seems that Olmert was willing to take a large personal risk in response to a goal he viewed as essential.

The General Staff also feels a similar obligation, and it is enough to see how many senior officers, particularly from the Air Force, have photos from the Auschwitz flyover on their walls, alongside other reminders of the Holocaust.

The so-called Begin Doctrine, which states that Israel will prevent with force if necessary, the introduction of nuclear weapons by enemy countries, was used against the Iraqi reactor in 1981, and also against the Syrians.

The international community made peace with the Israeli attacks, twice, since its actions did not cause a war that threatened oil markets.