Defense Secretary Leon Panetta is expressing new concerns about Iran's underground nuclear program, this time telling Washington Post columnist David Ignatius he's worried Israel may decide to attack it as early as this spring.

Traveling with the defense secretary in Brussels to cover his meeting with NATO defense ministers, Ignatius writes, "Panetta believes there is a strong likelihood that Israel will strike Iran in April, May or June.”

This is the first time we've heard such a specific timeframe. Fox News has previously reported concerns from former members of President Obama's national security team that a unilateral strike from Israel could occur sometime in 2012 and that Central Command has been planning for the possibility the U.S. could be drawn in.

One former government official who was involved in national security affairs agreed with Panetta's assessment on timing. "I think the likelihood of an attack is high, and the timing seems about right," the former official said, but added that Panetta "may have interrupted and delayed the timing by his disclosure."

Panetta and the administration have made clear in recent weeks that Iran would cross a "red line" by developing a bomb and that if that occurred, all options, including military action, would be on the table.

But Israel is less patient. It appears Israeli officials' red line would be crossed when Iran has the material to build a bomb. In other words, it appears they believe that by this spring Iran will have stockpiled enough highly enriched uranium to produce a nuclear warhead.

By that point, it will be too late for Israel to act alone. Unlike the United States, Israel does not have the capacity to strike Iran's hardened enrichment facilities 200 feet underground. That, along with Iran's arsenal of missiles that can reach Israel, add to Panetta's concerns that Israel is preparing to strike as early as this spring.

There are essentially two obvious methods for striking the underground facility that could be used. The first involves the Pentagon’s newly developed Massive Ordnance Penetrator, known simply as the MOP. The largest of its kind, it's a 30,000 pound bunker-busting bomb designed to hit underground targets. Yet in a recent interview with the Wall Street Journal, Panetta acknowledged the MOP has some shortcomings and needs further development to reach areas as deeply buried as Iran's nuclear facilities.

Since the MOP is the largest conventional weapon in the U.S. arsenal, the second option could involve using smaller-scale nuclear weapons. It's not likely a card the Obama administration would play, one that would make him the first president since Harry Truman to drop a nuclear bomb.

Another clue about Israel's intent was the sudden cancellation of long-planned joint U.S.-Israeli military exercises that would have culminated in live-fire drills this May. The Israelis apologized for postponing the exercises, and a Pentagon spokesmen said at the time of the cancellation the Israelis explained they needed to postpone in order to "assume optimum participation," suggesting their forces could be needed elsewhere.

Former Defense Secretary Robert Gates was asked Tuesday about getting into a new war, telling CNN’s John King, "If Iraq and Afghanistan has taught us anything in recent history, it is the unpredictability of war and that these things are easier to get into than to get out of."

He added, "This is, I think, one of the toughest foreign policy problems I have ever seen since entering the government 45 years ago."

Fox News' Jennifer Griffin contributed to this report.