Iran's supreme leader, Ali Khamenei, has said his country will press ahead with its nuclear programme.
Ayatollah Khamenei said the West was trying to stop Islamic progress
He was speaking a day before Tehran is scheduled to give its formal answer to an international proposal of incentives for Iran to halt uranium enrichment.
Iran had "made its own decision" and would "continue its path", state television quoted the leader as saying.
President Bush said he hoped the UN would impose sanctions if Iran did not stop enrichment by the end of August.
The UN Security Council has threatened unspecified economic and diplomatic sanctions if Iran does not stop enriching uranium by this deadline.
Iran is due to give its formal response on Tuesday to a proposal from Russia, China, France, Britain, Germany and the US aimed at ending the row.
The suggested package offers Iran a partial lifting of economic sanctions, together with assistance with civilian nuclear technology, in return for a suspension of uranium enrichment.
Enriched uranium is used as fuel for nuclear reactors, but highly enriched uranium can also be used to make nuclear bombs.
Iran maintains that its nuclear programme is for civilian use only.
"Arrogant powers, led by the United States, are fearful of progress of Islamic countries in various dimensions," Ayatollah Khamenei was quoted as saying on Monday.
There is concern in the West about the Arak heavy water plant
"Therefore... even though they know Iran is not seeking nuclear weapons, they are piling on the pressure to prevent our scientific progress."
President George W Bush said the US was waiting for Iran's formal response to the proposal, but urged the UN to move swiftly if Iran did not meet the 31 August deadline.
"In order for the UN to be effective, there must be consequences if people thumb their nose at the United Nations Security Council," he said.
But correspondents say Iran is fully aware that the UN will find it hard to agree on economic sanctions, with Russia and China opposed and with the West fearing the move would provoke higher oil prices.
Separately, the deputy head of Iran's Atomic Energy Organisation, Mohammad Saeedi, said on Monday that suspension of uranium enrichment was "no longer possible", according to the Iranian Fars news agency.
He also said a heavy water production plant at Arak would become operational "in the near future".
Heavy water is used to moderate the nuclear fission chain reaction in certain types of nuclear reactor, but can also be used to produce plutonium, which can be used to make nuclear bombs.
On Sunday, foreign ministry spokesman Hamid Reza Asefi said a resolution to the issue would have to come out of negotiations, but a halt to uranium enrichment was "not on the agenda".
He said Iran said it would give a "multi-faceted response" to the international incentives proposal, but did not elaborate.
Iran has said that it will respond to any possible sanctions with a painful response - which may involve a cut in its oil production.