The US Vice-President, Dick Cheney, has warned Iran's government it will not be allowed to obtain a nuclear weapon.
Mr Cheney said the US wanted to send Iran a clear message
Mr Cheney said the United States and the international community could not stand by as a "terror-supporting state" fulfilled its grandest ambitions.
He accused Tehran of practising delay and deception to buy time to pursue nuclear technology and of being an obstacle to peace in the Middle East.
Iran says its nuclear programme is for civilian purposes, not bomb-making.
The five permanent UN Security Council members and Germany will decide in November whether to impose tougher UN sanctions on Iran for continuing to enrich uranium.
'Delay and deception'
"The Security Council has twice imposed sanctions on Iran and called on the regime to cease enriching uranium," Mr Cheney said in a speech to the Washington Institute for Near East Policy on Sunday.
"Yet the regime continues to do so and continues to practice delay and deception in an obvious attempt to buy time."
Given the nature of Iran's rulers and the trouble it is causing in the Middle East, Mr Cheney said, the US and other nations could not "stand by as a terror-supporting state fulfils its most aggressive ambitions".
"The Iranian regime needs to know that if it stays on its present course, the international community is prepared to impose serious consequences," he added.
"The United States joins other nations in sending a clear message - we will not allow Iran to have a nuclear weapon."
There has been an escalation of criticism of Iran by the US administration over the past few weeks, the BBC's Sarah Morris in Washington says.
Last week, President George W Bush warned that a nuclear Iran could lead to another world war.
Mr Cheney's speech comes a day after the resignation of top Iran nuclear negotiator Ali Larijani.
Correspondents say his departure is a sign that President Mahmoud Ahmadinejad's tougher stance towards the West may be gaining influence within Iran.