Diplomats in Vienna today said that they were "very near" to building a united international diplomatic front against Iran's nuclear ambitions.

Foreign ministers from the E3 of France, Britain and Germany were gathered in the Austrian capital with counterparts from China and Russia for the latest round of talks to defuse the gathering crisis.

They were joined by Condoleezza Rice, Secretary of State for the United States, whose conditional offer to resume direct negotiations with Tehran after a 27-year silence - in return for the suspension uranium enrichment - is seen as heralding a potential breakthrough.

Manouchehr Mottaki, the Iranian Foreign Minister, today rejected the condition of the offer but said that he was otherwise prepared to enter talks with Iran's historic 'great enemy'.

"We support dialogue in a fair and unbiased atmosphere but we will not talk about our undeniable and legitimate rights, because this is the right of our people according to international laws and treaties," he told reporters.

President George Bush balanced yesterday's surprise offer of talks with a warning that he would press ahead with demands for economic and travel sanctions through the UN Security Council if Iran refuses to comply.

"It is going to be up to the Iranians to make their decision and if they choose not to verifiably suspend we have laid the groundwork for an effective international response," he said.

Margaret Beckett, the British Foreign Secretary, described the talks as a "a reopportunity for Iran and the international community to come to negotiated agreements and solutions."

Arriving in Vienna, she said that the US involvement would lend added weight to the summit: "We are all striving to reach a diplomatic solution," she added.

"The European side’s goal is to present a serious and substantial offer of co-operation, which demonstrates to Iran the benefits that would flow from compliance with the IAEA’s (International Atomic Energy Agency) successive resolutions, rather than the further isolation which would result from their failure to do so.

"I urge Iran to respond positively to this opportunity."

The other key issue is the US insistence that Russia and China sign up to sanctions if Tehran refuses to accept the package of incentives which is being finalised at the talks.

Proposed incentives include building Iran light-water reactors as part of a civil nuclear power programme. One compromise could be for Iran to keep spinning centrifuges that enrich uranium but leave them empty of the feedstock uranium gas.

China today welcomed the US offer to join talks with Iran but remains opposed to "arbitrary" sanctions. Russia, meanwhile, called on Iran to respond "constructively" to the US call. Both powers have strong trading links with Tehran and hold a veto on the UN Security Council.

A European diplomat said that today's meeting would, at best, revive the moment in the three-year cat-and-mouse dispute.

"Momentum is created. That’s the positive thing. But the momentum is not the solution. The hope is that it could initiate a positive reaction," he said.

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