Speaking at a meeting of Nato foreign ministers in Sofia, Bulgaria, on Thursday, Condoleezza Rice, the US secretary of state, said the security council must act if Iran fails to comply with demands.
Rice said it was "highly unlikely" Tehran would comply with demands that it suspend uranium enrichment.
"It goes without saying that the United States believes that, in order to be credible, the UN Security Council, of course, has to act," Rice told a news conference before a session of talks in which Nato allies were due to discuss possible steps.
Rice also said: "I look forward to discussing this with my colleagues and to I and others making that case, and I would certainly hope the security council is prepared to take some action.
"It cannot have its word and its will simply ignored by a [UN] member state," she said.
Speaking at the same meeting, Philippe Douste-Blazy, the French foreign minister, told reporters that France backed "a rapid and firm signal" to Tehran if it continued to ramp up its nuclear activities.
Earlier on Thursday, Mahmoud Ahmadinejad, the Iranian president, told a rally in northwest Iran that Tehran would pursue uranium enrichment in defiance of outside pressure.
Ahmadinejad remains firm on
pursuing uranium enrichment
Mohamed ElBaradei, chief of the International Atomic Energy Agency (IAEA), is expected to tell the security council and the agency's board on Friday that Iran has not stopped purifying uranium or fully answered IAEA queries from a month ago.
The West accuses Iran of seeking nuclear weapons under cover of a civilian programme.
Tehran denies the charge. It said this month it had processed uranium to the level used in power stations for the first time and planned large-scale enrichment.