Dick Cheney, the US vice-president, said in a speech on Tuesday to a pro-Israel lobbying group that Iran would not be allowed to have a nuclear weapon and said the issue may soon go before the Security Council.
"The Iranian regime needs to know that if it stays on its present course, the international community is prepared to impose meaningful consequences," Cheney told the American Israel Public Affairs Committee.
"We join other nations in sending that regime a clear message: We will not allow Iran to have a nuclear weapon."
He said the US "is keeping all options on the table in addressing the irresponsible conduct of the regime".
In the past the US had said it had no intention of using military force for the time being, but had declined to rule it out.
Also on Tuesday, Condoleezza Rice, the US secretary of state, said: "The United States has been very clear: The enrichment and reprocessing on Iranian soil is not acceptable."
But Rice shied away from warning of immediate UN sanctions after meeting Sergey Lavrov, the Russian foreign minister.
"We will see what is necessary to do in the Security Council," she said, adding that there was still time for Iran to change its ways.
At a joint news conference with Rice, Lavrov said there was no compromise in sight with Iran.
His statement signalled that Russia was backing away from what EU diplomats said was a proposal to let Iran do some atomic research if it agreed to refrain from enriching uranium on an industrial scale for seven to nine years.
Russia apparently abandoned the informal proposal after Western objection, but Lavrov denied that he had made any new proposal.
Condoleezza Rice and Sergei
Lavrov (L) in Washington, DC
"There is no compromise new proposal," Lavrov said.
Rice added: "The Russians did not tell us of any new proposal."
But EU diplomats said Russian officials informally raised the idea of a seven- to nine-year moratorium during consultations over the past week.
US, British, French and German objection came swiftly when word of the offer leaked on Tuesday.
Russia has been negotiating with Iran and has proposed enriching fuel on Russian soil for Iran's energy need.
The United States and the European Union want Iran to shelve all work to enrich uranium because of suspicions that Tehran is secretly trying to make nuclear weapons.
Rice and Lavrov later met George Bush, the US president, at the White House for about half an hour.
Asked afterwards if Russia would accept sanctions against Iran, which Moscow has seemed reluctant to do, Lavrov said: "Have you seen a proposal for any sanctions? This is a hypothetical question, yes?"