"We in the Pentagon, we plan for contingencies all the time and certainly there are options which exist" for dealing with the Iran nuclear threat militarily, Admiral Mike Mullen, chairman of the Joint Chiefs of Staff, told a forum at Columbia University in New York.
He said that his "worry about Iran achieving a nuclear weapons capability" is that other states in the region will seek nuclear arms of their own.
"There are those that say, 'Come on, Mullen, get over that. They're going to get it. Let's deal with it,'" Mullen said.
"Well, dealing with it has unintended consequences that I don't think we've all thought through. I worry that other countries in the region will then seek to, actually, I know they will, seek nuclear weapons as well. That spiral headed in that direction is a very bad outcome."
But he added: "I worry, on the other hand, about striking Iran. I've been very public about that because of the unintended consequences of that."
"The diplomatic, the engagement piece, the sanctions piece, all those things, from my perspective, need to be addressed to possibly have Iran change its mind about where it's headed."
The West accuses Tehran of seeking to produce atomic arms but Tehran says it aims only to generate electricity.
The five permanent U.N. Security Council members and Germany are accelerating negotiations on a new round of sanctions against Iran over its nuclear program, U.N. diplomats said last week.
A U.S. draft proposal provides for new curbs on Iranian banking, a full arms embargo, tougher measures against Iranian shipping, moves against members of Iran's Islamic Revolutionary Guard Corps and firms they control and a ban on new investments in Iran's energy sector.
(Reporting by Adam Entous; Editing by Jackie Frank)
