The State Department said reports that Iran has gone ahead with a threat to resume production of nuclear centrifuges were "disturbing," a matter of "deep concern" and a sign that the Islamic republic may not be trusted to fulfill its commitments.

"It's an issue that we remain deeply concerned about," deputy spokesman Adam Ereli said. "We view it as a direct challenge to the IAEA's call on Iran to suspend all enrichment-related and reprocessing activities.

"It certainly raises questions about other commitments Iran has made concerning its nuclear program," he told reporters.

"The reports we're seeing now I think are further evidence of a continued failure to abide by commitments," Ereli said. "Iran made a pledge to suspend the manufacture and assembly of centrifuges. They've gone back on that pledge. It's disturbing, and a matter of concern."

Earlier Wednesday at the Vienna headquarters of the International Atomic Energy Agency (IAEA), diplomats said Iran had removed seals the agency had placed on the centrifuges to ensure that Tehran was not using its civilian nuclear program as a cover for a secret weapons program.

"Seals on centrifuges monitored by the IAEA have indeed been broken," one Western diplomat told AFP. "The move indicates that Iran has resumed construction and assembly of centrifuges. But they have not resumed their enrichment operations per se."

The centrifuges are used to enrich uranium for use in nuclear power plants. But highly enriched uranium can also be used to make nuclear warheads.

Iran has denied US charges it is using its civilian nuclear energy program as a cover for clandestine atomic weapons development and on Wednesday a top member of parliament warned in Tehran that the government would delay ratification of an additional security protocol to the nuclear non-proliferation treaty (NPT) it had agreed to sign onto.

The protocol would give UN inspectors increased powers, including the right to carry out inspections without warning.

In addition to the NPT accord, Iran had agreed to seal its centrifuges after a visit by the British, French and German foreign ministers last October.

Earlier this month, Iran warned that it was not bound to this deal because the Europeans had failed to live up to their commitment to remove the issue from the IAEA's agenda.

Rather than take the Iranian dossier off the table, the European countries co-sponsored an IAEA resolution criticizing Tehran's failure to fully cooperate with the agency.

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