The tough rhetoric came after the regime also dismissed a personal appeal from UN atomic watchdog chief Mohamed ElBaradei to freeze uranium enrichment and calm suspicions it is seeking the atomic bomb.
"The enemy should know Iran is not comparable to any country in the world. Now we are much more powerful than before," senior cleric Ayatollah Ahmad Janati told worshippers at Friday prayers in Tehran.
"The United States is a decaying power. Don't be intimidated by their threats. They don't have the stamina to do anything," said the head of Iran's Guardian Council, a powerful political watchdog.
Iran announced this week that its scientists had successfully enriched uranium to make nuclear fuel, despite a UN Security Council demand for the sensitive work to be halted by April 28.
Iran says it only wants to generate atomic energy, but enrichment can be extended to make the fissile core of a nuclear warhead -- something the United States is convinced that "axis of evil" member Iran wants to acquire.
ElBaradei, the head of the International Atomic Energy Agency (IAEA), was rebuffed when he pressed the issue in Tehran on Thursday. He only said that talks would continue.
The IAEA chief must give a report at the end of April on Iranian compliance with the Security Council demand. After three years of investigations, he complained Iran's activities were "still hazy and not very clear".
In Washington, US Secretary of State Condoleezza Rice called for UN Security Council action and highlighted part of the UN charter that allows sanctions to escalate into military action.
"There is no doubt that Iran continues to defy the will of the international community," Rice said. "When the Security Council reconvenes, there will have to be some consequence for that action."
She suggested chapter seven of the UN Charter which sets out specific actions -- from sanctions to military action -- that can be taken when there is a threat to international peace or an act of aggression.
"One thing the Security Council has, and the IAEA does not have, is the ability to compel, through Chapter seven resolutions, member states of the UN to obey the will of the international system," Rice said.
"And I'm certain that we'll look at measures that could be taken to ensure that Iran knows that they really have no choice but to comply."
The US chief diplomat did not specifically call for any particular measure. US leaders this week said that reports of planned military action against Iraq were "wild speculation".
Representatives of the five permanent members of the Security Council plus Germany are to meet in Moscow Tuesday to discuss the crisis.
Although the United States has been prodding the council to take a tough stand against the Islamic republic, including possible sanctions, it has run into opposition from veto-wielding members Russia and China.
And Iran's hardline President Mahmoud Ahmadinejad also emphasised the mood in the regime that it is strong enough to deter any tough action.
"Today, thank God, the Iranian nation is a powerful one and we are going to have a dialogue with the world from a position of power," Ahmadinejad said in a speech in the northeast of the country.
"Everything we have is from God, and a few weaklings cannot stand against the Iranian people," he said.
In seeking to deter international action, Iran has been playing up its oil wealth, its military might in strategic Gulf waters and its influence across the region -- such as in Iraq, Lebanon and the Palestinian territories.
And in Tehran Friday, the regime's supreme leader Ayatollah Ali Khamenei was also opening an international conference on supporting the Palestinian fight against Israel. The gathering includes representatives from the militant groups Hamas, Islamic Jihad and Hezbollah.
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