John Bolton, under secretary of state for arms control and international security, said the announcement was a direct violation of Iranian pledges to the so-called "EU three" and proof of Iran's intent to reprocess uranium as part of a covert nuclear weapons program.

Bolton, a noted hawk within President George W. Bush's conservative administration, said Iran was in violation of its commitments to the International Atomic Energy Agency (IAEA), was flagrantly defying the will of the international community and that the matter should be referred to the UN Security Council for possible imposition of sanctions.

"This is an act of defiance of the IAEA Board of Governors, it is a thumb in the eye of the international community," he told the House International Relations Committee.

"It has been our view, it remains our view (and) Iran's action today confirms our view that its nuclear weapons program is a threat to international peace and security and should be referred to the UN Security Council," Bolton said.

Bolton did not say how Washington had been informed of the Iranian announcement on centrifuges but said it appeared to be a direct response to a harshly critical assessment of Tehran's cooperation with the IAEA that was issued by the agency's governing board last week.

Iran's top national security official and nuclear negotiator Hassan Rowhani had said that in response to the IAEA criticism, Tehran could resume uranium enrichment activities but other officials suggested resuming the assembly of centrifuges was a more likely reaction.

There was no official Iranian announcement about the resumption of centrifuge production.

Such a move would violate Tehran's February agreement with Britain, France and Germany which had in October reached a deal with Iran for it to halt uranium enrichment.

Bolton said Washington had always suspected that the Iranians had not completely stopped its centrifuge production, but that their statement to the Europeans on Thursday confirmed the US view.

He also said the move was a clear sign that Iran intended to resume uranium enrichment for its alleged weapons program.

"They have not, at least at this point, said that they would resume actual enrichment activities, but it seems to me perfectly obvious that Iran is not producing components for uranium centrifuges to use them as knickknacks in Iranian living rooms," Bolton said.

On Monday, supreme leader Ayatollah Ali Khamenei said it was essential for the Islamic republic to master the nuclear fuel cycle, but again denied the country was seeking to develop nuclear weapons.

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