WASHINGTON - The top US military officer says he has a plan to attack Iran if needed to prevent it from getting nuclear weapons, but is "extremely concerned" about the possible repercussions of such a strike.

Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, said military action against Iran could have "unintended consequences that are difficult to predict in what is an incredibly unstable part of the world."

But, speaking on Sunday's "Meet the Press" program on NBC, Mullen said allowing Iran to develop a nuclear weapon was also unacceptable.

"Quite frankly, I am extremely concerned about both of those outcomes," he said.

Mullen held out hope that a combination of international diplomatic efforts and sanctions against Iran would lead Tehran to suspend a nuclear enrichment program that many believe is a clandestine bid to develop nuclear arms.

"I am hopeful (it) works," he said.

At the same time, though, he said "the military options have been on the table, and remain on the table".

"I hope we don't get to that, but it's an important option and it's one that's well understood," he added.

Asked if the military has a plan to strike Iran, Mullen replied, "We do".

He did not elaborate.

Iran insists its uranium enrichment program is for peaceful purposes only.