With European and US officials scornful of the offer and determined to report Iran to the UN Security Council, Russia said Tehran must halt the atomic fuel research it resumed last week in defiance of world powers if it wanted negotiations.

Sean McCormack, the US State Department spokesman, said: "Talk is cheap. What they're engaged in is stirring up a lot of chaff. They've started up their diplomatic fog machine here."

A letter to restart talks intended at allaying fears that Tehran is pursuing development of an atomic bomb, written by Javad Vaeedi, deputy head of Iran's Supreme National Security Council, was labelled as "vacuous" by a senior British official.

Russia, which has taken a tougher stance against Iran in recent weeks despite strong commercial and diplomatic ties, told Tehran to restore the basis for negotiations by reversing last week's move.

Sergei Lavrov, the Russian foreign minister, said: "Talks pre-suppose an obligation. The Iranian obligation was to stick to the moratorium. Now Iran (has departed from) the moratorium on scientific research."

Western suspicions

Britain, France and Germany called off the talks after Tehran removed UN seals on uranium enrichment equipment, deepening Western suspicions that it is seeking nuclear arms.

Sergei Lavrov: Talks pre-suppose
an obligation

Iran insists it seeks only a peaceful energy programme. Washington and its EU allies say it is time the UN nuclear watchdog agency sent Iran's case to the UN Security Council, which could eventually decide to impose sanctions on Iran.

China has demurred, saying it would like talks between Iran and the EU trio to resume, but has not said it will try to block any move to report Iran to the Council.

Zhou Wenzhong, China's ambassador to the United States, said: "Efforts through diplomatic channels need to be given another chance."

Russia, while sharing China's opposition to UN sanctions on Iran, has moved closer to the West's view on referral.

And Washington predicted that a 2 February meeting of the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), agreed on by the United States, Britain, France, Germany, Russia and China, would report Iran to the Security Council.

McCormack said: "We're headed to the Security Council right now."

Scepticism

Many non-proliferation experts and diplomats are sceptical the Security Council could stop Iran building a bomb if that is Tehran's goal.

US intelligence in 2005 said Iran could produce a weapon in about a decade, although most estimates predict it could do so far sooner with bomb-grade fuel from a foreign supplier.

Chances of a UN Security Council
referral rattled oil markets

An Iranian source in Vienna said Iran had written to the EU trio proposing that talks restart immediately, and saying Tehran was ready to "remove existing ambiguities regarding its peaceful nuclear programme through talks and negotiations".

Despite Tehran's call for talks, Ali Asghar Soltaniyeh, Iranian representative to the IAEA, said the decision to resume nuclear fuel research was "irreversible".

Oil unrest

The chances of Security Council referral have rattled oil markets because of worries about either an eventual embargo or the prospect Iran could retaliate by removing all or part of its daily crude oil sales of 2.4 million barrels amid stretched global supplies.

Any Security Council action would need the consent of Russia and China, both wary of jeopardising their major economic interests in Iran.

Iran is a major oil supplier for China. Russia has a $1 billion stake in building Iran's first atomic reactor.