The British government, which is spearheading the EU-Iran talks along with France and Germany, also rebuffed an Iranian call for immediate new proposals from the so-called E3, saying it would make new proposals in a week's time.

The Foreign Office said it was seeking "clarification of Iran's intentions" and urged Tehran to avoid any unilateral move which "would make it very difficult to continue" the negotiations with the European Union.

"We have received reports that the Iranians have decided to restart their uranium conversion facility at Isfahan," suspended last November under an accord struck in Paris, said Britain, which currently holds the EU presidency.

"This would be an unnecessary and damaging step by Iran," said the statement, adding that the E3 foreign ministers and EU foreign policy chief Javier Solana had written to Iran's top nuclear negotiator Hassan Rowhani.

Meanwhile in Paris, a European diplomat said that the latest Iranian threats amounted to "barely acceptable pressure" on the EU, and that "a generous offer" was being prepared by the bloc.

"We ask the Iranians to study this offer and not to do anything contrary to the Paris accord and the resolutions" of the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said the diplomat.

The Iranian threat "is barely acceptable pressure that leads us to express our surprise and our concern," said the diplomat, who asked to remain anonymous.

The EU letter confirmed that "full and detailed proposals would be given to Iran in a week's time, in accordance with the decisions at the E3/Iran ministerial meeting in Geneva in May," said the British foreign ministry.

The statement came after Iran warned Sunday that it would resume sensitive nuclear work within 24 hours if the EU failed to submit proposals aimed at ending a long-running crisis over its nuclear programme.

The move has dramatically raised the stakes in the standoff and risks seeing Iran hauled before the UN Security Council for possible sanctions, a persistent demand of the United States, which accuses Iran of seeking atomic weapons.

In Tehran, a foreign ministry spokesman said Iran would inform the IAEA on Sunday or Monday of the resumption of some uranium conversion work.

"The time limit (on the suspension of such activities) has passed and public opinion cannot wait any longer," said spokesman Hamid Reza Asefi.

"If the Europeans submit their proposals by 5:00 pm (1230 GMT) we will examine them, if not we will resume some of our activities in Isfahan tomorrow," nuclear negotiator Ali Agha Mohammadi told AFP.

But he added: "Our position is that we want to pursue the negotiations with the Europeans."

The conversion process, carried out in Iran at a facility in the central city of Isfahan, changes uranium ore into the uranium gas that is the feedstock for enrichment.

"We are seeking clarification of Iran's intentions," the Foreign Office statement said.

"We urge them not to take any unilateral step which would contravene the Paris agreement as that would make it very difficult to continue with the E3/Iran negotiations," it said.

Should the Iranians persist, "we will as a first step consult urgently with our partners on the board of the IAEA, which is monitoring Iran's nuclear plans," it said.

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