"Today, with the authorization given by the IAEA to its inspectors (to supervise the action) ... seals from a number of research centers were removed," said Mohammad Saidi, deputy head of Iran's Atomic Energy Agency said, referring to the International Atomic Energy Agency.

"As of today these centers resume their activities," Saidi told reporters. "The research will be carried out in all the centers that we told the IAEA about, and we will restart our work.

"The production of nuclear fuel is still in suspension and we hope to reach a conclusion over it in the near future, and also reach a clear agreement with the Europeans in this regard," Saidi said.

EU officials said the resumption violates an agreement to refrain from nuclear activities.

"This is very much a step in the wrong direction," said Cristina Gallach, spokeswoman for EU foreign policy chief Javier Solana, according to Reuters news service. "We are extremely concerned and consultations are taking place (within the EU) to coordinate a response."

Asked whether the move increased the likelihood of a referral to the UN Security Council for possible sanctions, Gallach said that further steps would have to be discussed.

Solana and ministers from Britain, France and Germany -- the so-called "EU3" states leading negotiations with Iran aimed at persuading it to shelve its nuclear program -- are due to meet in Berlin on Thursday to consider their next step.

In Vienna, the IAEA confirmed that Iran removed seals at its Natanz atomic research facility under the supervision of its inspectors.

US mulls Security Council referral

Tehran announced last week it would restart research into the nuclear fuel cycle despite international calls to keep the voluntary suspension of such work in place.

The United States, which accuses Tehran of seeking to build nuclear weapons, reiterated Monday that Iran may be referred to the UN Security Council over its action. But Russia said dialogue was still the only way forward.

On Tuesday, Moscow said its offer to enrich uranium jointly with Iran at a site on Russian territory remained valid. Talks on the compromise proposal are due to resume next month after breaking off at the weekend without agreement.

"We confirmed our proposal, it remains on the negotiating table, and if our Iranian colleagues are interested we are ready to develop a joint plan to resolve the entire Iranian energy issue," Deputy Foreign Minister Sergei Kislyak told Interfax news agency.

ElBaradei is losing patience

IAEA chief Mohamed ElBaradei said Monday he was "losing patience" with what he called Iran's lack of transparency.

The international community has already warned that "the next step would be a referral to the Security Council" if Tehran failed to keep its international obligations, the White House said Monday.

Spokesman Scott McClellan told reporters that Iran must maintain a total suspension of activities linked to uranium enrichment, which produces fuel for nuclear power reactors but can also be used to make atomic bombs.

"The international community has growing concerns about the regime's pursuit of nuclear weapons under the guise of a civilian nuclear programme," McClellan said.

Iran remains defiant

Iran's supreme leader, Ayatollah Ali Khamenei, voiced defiance on Monday, saying Tehran would not give up its nuclear program.

"The Islamic Republic of Iran will not give up its undeniable rights to peaceful nuclear technology, which has been achieved by the talented youth of the country," Khamenei said.

Iran has been trying to draw a distinction between research into the fuel cycle and actual production of enriched uranium, which can be used as fuel in civil reactors or, in highly enriched form, as the explosive core of an atom bomb.

The removal of the seals came after talks between Russia and Iran on a proposed compromise to end the row over uranium enrichment broke off without result Sunday, although they are to resume on Feb. 16.

Moscow has been proposing that Tehran carry out uranium enrichment on Russian territory to allay Western fears that the technology could allow Iran to produce a nuclear bomb.