The United States and Europe launched a co-ordinated drive on Friday to press Iran to abandon its most sensitive nuclear activity, which experts say could enable it to make the bomb. Washington was set to announce it would offer Iran economic incentives - a start to World Trade Organisation membership talks and access to civil aircraft and spare parts - in a major policy shift requested by the Europeans.
In return, Britain, France and Germany said they would haul Tehran before the UN Security Council if it resumed uranium enrichment and nuclear reprocessing activities.
The joint strategy was a first fruit of US President George W. Bush's trip to Europe last month and appeared to bridge, at least for now, years of transatlantic argument over whether to engage or isolate the Islamic republic.
The three European heavyweights told EU partners in a letter that "progress is not as fast as we would wish" in talks they began last December to persuade Tehran to end its most sensitive nuclear work in return for economic and political benefits.
The United States accuses Iran of secretly trying to develop nuclear arms. Tehran says its programme, long concealed from the UN nuclear watchdog, is purely for civilian energy purposes.
But chief Iranian negotiator Hassan Rohani insisted in an interview published on Friday that Iran would not give in to Western demands that it scrap efforts to complete the fuel cycle, which could help it make bombs.
The EU3 said that if Iran continued its suspension of all enrichment and reprocessing activities and co-operated fully with the UN's Vienna-based International Atomic Energy Agency, they believed the issue could be resolved at that level.
"If on the other hand, despite our efforts Iran does not do so, then as has been implicit in the agreements reached with Iran and well understood by all concerned, we shall have no choice but to support referring Iran's nuclear programme to the UN Security Council," the letter obtained by Reuters said.
Iran threatened last week to resume uranium enrichment and scrap the talks if it was taken to the Security Council, an attitude which Western diplomats said appeared to be a bluff to assuage intense national pride on the nuclear issue.
The EU3 letter made no mention of the possibility of UN sanctions against Tehran. Diplomats said Washington had pressed for such a reference but the EU3 had replied that it would be up to the Security Council to decide what action to take.
"We are united in our determination that Iran should not acquire a nuclear weapons capability," the three countries said.
The EU3 letter gave a broad outline of the talks in working groups on nuclear issues, political and security matters and technology and economic co-operation, but contained no surprises.
As expected, it said the nuclear part was the toughest, since "both sides have strongly held positions on this difficult issue, which remains at the core of the negotiations, and discussions continue".
The letter confirmed that the EU3 had offered to "back the modernisation of Iranian civil aviation" - an apparent reference to plans to sell Tehran Airbus planes and spare parts with US blessing.