An Israeli strike on Iranian nuclear facilities would be a high-risk move that could destabilize the Middle East, the top U.S. military officer said Wednesday.

At a Defense Department news conference, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, refused to say what Israeli leaders told him during meetings last week about any intentions to strike Iran.

But asked whether he was concerned Israel would strike before the end of the year, he said: "This is a very unstable part of the world and I don't need it to be more unstable."

The U.S. military is severely strained already by wars on two fronts — the nearly seven-year-old campaign in Afghanistan and more than five years in Iraq.

"Opening up a third front right now would be extremely stressful on us," Mullen said. He added, however, that if a conflict began, he believes that Iran has the capability to interrupt ship traffic through the strategic Strait of Hormuz.

He would not say whether the U.S. Navy is stepping up its presence in the region, but asserted that Iran would not have the ability to sustain a blockade in the Strait.

The Bush administration and other world leaders allege Iran is seeking to produce nuclear weapons and Iran says its nuclear program is aimed only at generating electricity.

"I believe they're still on a path to get nuclear weapons and I think that's something that needs to be deterred," Mullen said, adding that it should be done through diplomatic, financial and economic actions by the U.S. and other nations.

But, he added, "I think that just about every move in that part of the world is a high-risk move."

Mullen would not talk about military intelligence that some suggest indicates that Iran will be able to acquire surface-to-air defense capabilities and have the ability to enrich enough uranium to create a nuclear bomb by the end of the year.

In a news conference earlier in the day, President George W. Bush also was asked about increasing speculation that Israel will launch a military strike, saying that all options are on the table but that military action would not be his first choice.

"I have made it very clear to all parties that the first option ought to be solve this problem diplomatically," Bush said. "And the best way to solve it diplomatically is for the United States to work with other nations to send a focused message — and that is, you will be isolated, and you will have economic hardship, if you continue to enrich" uranium for a bomb.