"We continue to call on Iran to get back to the negotiating table and act in good faith. We've made that very clear. We have deep concerns about Iran's behavior, given their history," said spokesman Scott McClellan.

Washington says Tehran has been using a civilian nuclear program as cover for efforts to get atomic weapons. Iran denies the accusation.

The United States has backed diplomatic efforts by Britain, France and Germany to defuse the standoff, but has increasingly warned that it may be time for the UN Security Council to consider sanctions against Iran.

"The international community is growing more and more concerned about Iran's refusal to come back to the talks," McClellan said, pointing to a recent meeting of the UN nuclear watchdog agency.

"You saw at the International Atomic Energy Agency that a majority now supports sending Iran to the Security Council if they don't come back to the table and negotiate in good faith.

"And that is certainly an option that is on the table, and if Iran does not change its behavior and get back to the negotiating table, it is an option that is available to the international community," the spokesman said.

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