The top U.S. military officer says there is a narrow window of time in which to prevent Iran from acquiring nuclear weapons and also to avoid a military strike on its nuclear facilities.  The Chairman of the Joint Chiefs of Staff, Navy Admiral Michael Mullen, said in Washington Tuesday dialogue is crucial but that time is running out.

Admiral Mullen said he is very concerned about a state sponsor of terrorist groups, such as Iran, acquiring nuclear weapons.  He said it would be "incredibly destabilizing" and could cause a regional arms race. 

But Mullen said he is equally concerned about a possible pre-emptive military strike on Iran's nuclear facilities, such as Israel is reported to be contemplating.  He said such a move could have a variety of unintended and potentially deadly consequences.

"I worry a great deal about the response of a country that gets struck and the vulnerabilities that regional countries have that are great friends of ours, their populations," Mullen said.  "And then what's next?  And then how does it end up?  Does it, in fact, get contained or does it expand?"

Admiral Mullen said the way to avoid a nuclear Iran and a military strike against it is through dialogue, like the renewed contacts that President Barack Obama has proposed.

"There is a great deal that certainly depends on the dialogue and the engagement.  And I think we need to do that, with all options remaining on the table - including, certainly, military options," he said.

Mullen would not elaborate on when the United States might use a military option against Iran or what it might entail.  But he said various estimates suggest that Iran is between one and three years away from developing a nuclear weapon.

"That gets back to the criticality, in my view, of solving this before Iran gets a nuclear capability or that anyone would take action to strike them.  And I think that window is a very narrow window," Mullen said.

Admiral Mullen spoke to the Center for Strategic and International Studies in Washington Tuesday, just hours after returning from Moscow, where he participated in the U.S.-Russia summit on Monday.

He said the summit preparations included discussions of how the United States and Russia can cooperate on the Iranian issue.  He would not provide details, except to say that U.S. officials urged Russia not to deliver to Iran a sophisticated air defense system it wants, which the admiral said would be "a game changer" in Middle Eastern security.

Experts say a firm plan to activate the system would put more pressure on Israel to attack Iran's nuclear facilities before improved air defenses made an air strike more difficult.