US President George W Bush said Iran had "weeks, not months" to agree to freeze enrichment, which can be used to make reactor fuel or a nuclear weapon, or face UN Security Council punishment.
His comments came as a cleric close to Iran's supreme leader said Tehran would not suspend its nuclear program of uranium enrichment, a day after the UN nuclear watchdog said it had actually accelerated the sensitive work.
"We must have uranium enrichment between 3.5 to 5 per cent and they have to accept it," Ayatollah Ahmad Janati told worshippers at prayers in the capital.
An Iranian official, quoted by the student news agency ISNA, confirmed the accelerated activity.
"Iran has started another stage of injecting hexafluoride gas into centrifuge machines," he said.
Iran considers uranium enrichment to be its right under the nuclear Non Proliferation Treaty (NPT). But Western powers, who suspect Iran's nuclear program has military ambitions, want it to suspend the practice.
Iran's supreme leader, Ayatollah Ali Khamenei, has not yet responded publicly to the offer from the permanent members of the security council and Germany, aimed at pushing Teheran to suspend uranium enrichment.
But world leaders were quick to warn Teheran it only had a few weeks to respond to the offer, which also includes a number of incentives for its nuclear program and industry as part of a "carrot and stick" approach.
"We've given the Iranians a limited period of time…you know, weeks not months… to digest a proposal to move forward. And if they choose not to verifiably suspend their program, then there will be action taken in the UN Security Council," said President Bush.
French President Jacques Chirac and British Prime Minister Tony Blair also sent out a strong message to Iran to halt its controversial nuclear program as they met for talks in Paris.