Speaking on the British television channel ITV, Rice recalled that Washington had supported diplomatic efforts by France, Britain and Germany to persuade Iran to give up its programme of uranium enrichment.

"The United States has been clear that we are supportive of what the Europeans are trying to do in giving the Iranians an opportunity to show the world that they are prepared to live up to their obligations and of course we retain the possibility of referral to the (UN) Security Council," Rice said on the fringes of a meeting here to discuss Palestinian reforms.

Asked whether she thought military action against Iran was possible, she said the US "never categorically rules out anything but we are in a state in which diplomacy has time to work, in which we have many other diplomatic arrows in our quiver."

The UN atomic watchdog agency said Tuesday in Vienna Iran had refused to cooperate in key areas with UN experts investigating possible atomic weapons work, including blocking a follow-up visit to the Parchin military facility, where Washington charges Tehran is simulating testing of atomic weapons.

Britain, France and Germany have been trying to persuade Tehran to permanently abandon its capacity to produce enriched uranium -- which can be directed to both civil and military uses -- in return for a package of incentives.

The diplomatic effort is aimed at securing "objective guarantees" that Iran will not produce nuclear weapons.

The United States charges that Iran has a clandestine nuclear weapons program and wants the IAEA to bring Tehran before the UN Security Council for possible sanctions.