Undersecretary of State Nicholas Burns said Tehran has "crossed the international red line" with its activities to enrich uranium, and that unless Iran suspends all nuclear activities, the Security Council must get involved.
Burns did not say what the United States might ask the Security Council to do about Iran.
Washington has accused Iran of secretly trying to develop nuclear weapons, a charge that Iranian officials have consistently denied.
In separate comments on March 6 -- the same day that the UN's International Atomic Energy Agency (IAEA) board convened to discuss referring Iran to the Security Council for possible sanctions -- U.S. State Department spokesman Tom Casey said Iran should not be allowed to carry out even small-scale uranium-enrichment work.
Earlier the same day, the head of the UN nuclear watchdog agency, Muhammad el-Baradei, said he was still hopeful an agreement could be reached to ease the Iranian nuclear standoff.
(compiled from agency reports)