The top U.S. military official said Sunday he believes Iran has enough fissile material to build a nuclear weapon, but Defense Secretary Robert Gates said the Islamic Republic is a long way from having a bomb.

Last week, the International Atomic Energy Agency revised its assessment of Iran's nuclear capabilitie, saying it was wrong in earlier reports about Iran's ability to enrich enough uranium to make anuclear weapon.

Adm. Mike Mullen, the chairman of the Joint Chiefs of Staff said, "We think they do, quite frankly," when asked Sunday about Iran's capacity.

"Iran having a nuclear weapon I've believed for a long time is a very, very bad outcome for the region and for the world," Mullen said.

But Gates said on NBC's "Meet the Press" that the Iranians are not close to getting a weapon at this time.

"They're not close to a stockpile, they're not close to a weapon at this point and so there is some time," he said when asked whether Tehran could be deterred from pursuing its weapons effort.

Gates said the U.S. will continue to pursue sanctions on Iran while also howing an opportunity to engage with Europe as a means to "walk away from that program."

"Our chances of being successful, it seems to me, are a lot better at $35 or $40 oil than they were at $140 oil because there are economic costs to this program, they do have economic challenges at home," Gates said.

Mullen said he has not been given "any instructions one way or the other" on whether to continue working on an anti-ballistic missile shield that has been in development and was to be deployed in East Europe.

"There are an awful lot of reviews that are ongoing under President Obama, and there's an awful lot on the -- on all of our plates. So that's a review that will, I think, take place. And over time, that's much more a policy area than it is mine, per se," he said.

Mullen said that the U.S. is also watching North Korea closely, but no decisions have been made on whether to rrespond to their preparations for a test missile launch.

"The president's made no decision. Secretary Gates and I have made no recommendations. But it's -- it's an area that we watch with great concern. And I would hope that North Korea would not be provocative," he said.

Mullen spoke on "FOX News Sunday" and CNN's "State of the Union."