The chairman of the Joint Chiefs of Staff stressed on Sunday that diplomacy is the United States' best offense against Iran's nuclear program, but he hinted that last-resort military options do still "exist."

"We in the Pentagon, we plan for contingencies all the time and certainly there are options which exist" to handle Iran using military force, Adm. Mike Mullen said during an event this weekend at Columbia University, as first reported by Reuters.

However, Mullen soon noted, "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," he said.

Mullen's remarks arrive a day after reporters for The New York Times uncovered a secret memo from Defense Secretary Robert Gates sent earlier this year to the president and top security staff. The secret communication expressed serious concerns that the United States lacked a comprehensive means of dealing with Iran's burgeoning nuclear program.

However, the White House has since downplayed the document. Gen. James Jones, the National Security Adviser, later told the Times, "The fact that we don’t announce publicly our entire strategy for the world to see doesn’t mean we don’t have a strategy that anticipates the full range of contingencies — we do.”

Still, the news has nonetheless called into question how far the United States will allow Iran to proceed with its nuclear energy program before it seeks more forceful consequences from the U.N. Security Council and other bodies.

Already, more than 300 House members are calling on the president to impose tougher sanctions on the Middle Eastern state, and both the House and Senate will confer next week on sanctions bills they passed earlier this year.