The sources said Iran had removed the seals that 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.
In Tehran, the deputy head of the Iranian parliament's foreign policy and security commission, Mohamoud Mohammadi, said the parliament will not ratify an additional security protocol to the nuclear non-proliferation treaty (NPT). 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.
But earlier this month, it 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 agenda of the International Atomic Energy Agency, the UN's nuclear watchdog.
"Seals on centrifuges monitored by the IAEA have indeed been broken," a 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.
However, he said the removal of the seals at the Natanz nuclear center 250 kilometers (150 miles) south of Tehran appeared to be a "kind of maneuvering, maybe a symbol of defiance" ahead of renewed talks with the three European powers in London or Paris in coming days.
Rather than take the Iran dossier off the table, the European countries co-sponsored an IAEA resolution criticizing Tehran's failure to fully cooperate with the agency.
The European governments said their promise was conditional on whether the IAEA's director, Mohamed ElBaradei, was satisfied with Iranian cooperation -- which he was not.
The United States has been putting pressure on the IAEA to send the Iran dossier to the UN Security Council for possible adoption of sanctions against Iran.
Diplomatic sources said Tehran may have decided to put the centrifuges back into operation at this time because the Europeans were not together on the issue, and the United States was distracted by its presidential election campaign.