Iran has threatened to hit back with 'an even more decisive strike' if the U.S. bombs its nuclear sites.
The warning from the head of the Revolutionary Guards came in response to tough American sanctions against Tehran.
The sanctions are designed to cut off financial support for the Revolutionary Guards, the elite Iranian force designated a 'proliferator of weapons of mass destruction' by Washington.
But the force's new chief, General Mohammad Ali Jaafari, insisted that Iran was ready to defend itself.
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Asked whether he thought the Americans would go beyond sanctions and use firepower to curb Iran's nuclear programme, he scoffed: 'Enemies cannot do a damn thing and these words are only excessive rhetoric. I don't consider them a threat.'
He was speaking after U.S. Secretary of State Condoleezza Rice strongly defended the sanctions, which are the toughest America has imposed on Iran since the 1979 revolution.
They ban U.S. dealings with 20 Iranian organisations linked to the Revolutionary Guards - and step up pressure on international banks to cut any ties with those firms.
Insisting the measures were needed to 'confront the threatening behaviour of the Iranians', she added: 'The international community cannot just sit idly by.
'A nuclear weapon in the hands of the Iranian regime would be deeply destabilising in the world's most volatile region.'
China and Russia have already come out against the sanctions, with president Vladimir Putin of Russia comparing the Bush administration to 'mad people wielding razor blades'.
Dr Rice tried to play down any rift with Russia, saying neither wanted a nuclear armed Iran.
'After all, Moscow is a lot closer to Iran than the United States,' she said.