United States President Barack Obama's national security advisers are considering a range of options to curb Iran's nuclear programme, among them military strikes, if diplomacy and sanctions fail, Pentagon officials say.
Defence Secretary Robert Gates said he had identified "next steps in our defence planning process" that would be reviewed by decision-makers in the coming weeks and months.
Admiral Mike Mullen, chairman of the US military's Joint Chiefs of Staff, said on Sunday that the military options available to Mr Obama would go "a long way" to delaying Iran's nuclear progress but may not set the country back in the long term. He called a military strike his "last option" right now.
The New York Times reported on Saturday that a memo by Mr Gates in January was meant as a warning to the White House that the US lacked an effective strategy to curb Iran's steady progress toward nuclear capability.
However, Mr Gates said the memo was not intended as a "wake-up call" or received as such by the president's national security team.
"Rather, it presented a number of questions and proposals intended to contribute to an orderly and timely decision-making process."
At a summit in Iran, Supreme Leader Ayatollah Ali Khamenei labelled the US an "atomic criminal", while Iranian President Mahmoud Ahmadinejad said an independent body should be set up to oversee nuclear disarmament.
Last week, the US hosted a conference of 47 nations to discuss nuclear security, to which Iran was not invited.