Jack Straw, the Foreign Secretary, warned the Iranians today that they were "pushing their luck" by removing the seals placed on a nuclear enrichment facility by the International Atomic Energy Agency two years ago.

The removal of the seals despite a moratorium on nuclear activity raises the possibility of Security Council resolutions against Tehran, which claims that it is simply trying to develop a nuclear power industry to meet its burgeoning demand for electricity rather than nuclear weapons.

The United States accused Iran of showing disdain for international diplomacy. Gregory Schulte, the chief US representative at the IAEA, said: "The regime continues to chose confrontation over co-operation, a choice that deepens the isolation of Iran and harms the interests of the Iranian people."

He added: "Iran is taking another deliberate step toward enrichment, which creates the material for nuclear bombs."

Russia, which has been trying to broker a deal in international negotiations and is helping Iran build a nuclear power station, also expressed its concern, but said that it would try to ensure that a moratorium continued on the enrichment of uranium - which can make fuel both for nuclear reactors and for a bomb.

Mr Straw said that he endorsed a comment by President Jacques Chirac of France that Iran would be making a "serious error" if it did not co-operate with the international community. "They are pushing their luck," he told reporters.

Later, in a statement at the Foreign Office, Mr Straw said that he would meet his French and German counterparts - the trio of foreign ministers negotiate with Iran on behalf of the European Union - on Thursday to discuss their response and the possibility of Security Council sanctions.

But he added: "Military action is not on our agenda, and I don’t believe it’s on anyone else’s agenda."

The deputy head of the Atomic Energy Organisation of Iran, Mohammad Saeedi, told a press conference that officials of the International Atomic Energy Agency had authorised the seals’ removal at the Natanz facility last night.

"Today, with the powers delegated by IAEA inspectors in Iran, some of the seals that are in the field of only research were removed, and research facilities resume their work," Mr Saeedi said.

Mr Saeedi indicated that Iranians had actually taken off the seals when he said they were removed "in the presence of IAEA inspectors."

Natanz is an underground facility in central Iran, which was unknown to UN non-proliferation inspectors until an Iranian exile group revealed its existence in August 2002.

In Vienna, an IAEA spokeswoman, Melissa Fleming, said that the Iranians had removed the seals at the Natanz facility in the presence of IAEA inspectors. Ms Fleming said that the UN agency’s 35-nation board of governors would be informed later today about what the Iranians planned to do with the unsealed equipment.

IAEA inspectors had arrived in Tehran on Saturday to remove the seals put on the nuclear research sites more than two years ago.

Mr Saeedi stressed that Iran was not resuming the production of nuclear fuel, a process that would involve uranium enrichment.

"What we resume is merely in the field of research, not more than that," he said. "We make a difference between research on nuclear fuel technology and production of nuclear fuel. Production of nuclear fuel remains suspended."

Mr Saeedi did not specify which nuclear facilities would resume operations, saying that the equipment from which the seals were removed was "a confidential issue between us and the IAEA".

Senior US, German and British officials criticised Iran yesterday for its declared intention to resume nuclear work that it had suspended as a goodwill gesture during negotiations on its nuclear programme.

In Washington, the State Department spokesman Sean McCormack said that each of the permanent members of the UN Security Council had told Iran to drop its plans or risk being hauled before the Council for possible sanctions.

"We are working very closely with Russia, China and France and Britain on sending a clear message to the Iranians," Mr McCormack said.

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