Foreign ministry spokesman Hamid Reza Asefi said Iran had been negotiating with the European Union's big three after they sent a letter apparently offering technical assistance in return for Tehran's full compliance with International Atomic Energy Agency (IAEA) demands it prove it is not secretly developing nuclear weapons.

"Since the foreign ministers of Germany, Britain and France have sent us a letter showing their inclination to cooperate with us, we have decided on a constructive dialogue," Asefi said, adding that talks have been going on "for several days".

"It is possible in the coming days that this will reach a higher level -- so we have invited the three foreign ministers to Iran to give their points of view," the spokesman said.

On Friday, an Iranian government source said Jack Straw, Dominique de Villepin and Joschka Fisher could visit Tehran as early as next week in an unprecedented joint visit aimed at capping a behind-the-scenes diplomatic bid to resolve a brewing crisis over Iran's nuclear programme.

According to diplomatic sources, Britain, France and Germany have been secretly negotiating a deal with Iran's clerical leaders.

The EU states are insisting Iran accept strict controls on its nuclear activities by signing and implementing an additional protocol to the Non-Proliferation Treaty (NPT) that would allow surprise inspections by the IAEA of suspect sites.

According to a Western diplomatic source, if Tehran accepts all the IAEA's demands, Germany, France and Britain "may favourably consider the possibility of supplying (Iran) with technology, even nuclear technology, as well as nuclear fuel."

The source here said senior representatives of the three countries had visited Tehran in recent days to "resolve this crisis peacefully".

In an interview with the BBC on Friday, IAEA head Mohamed ElBaradei spoke of "discussions between Iran and some of the European countries to try to see whether in fact, to defuse the security concern, Iran might get assurance of supply" of nuclear fuel.

"I hope ... that such settlement will be possible in the future. I think it would be a win-win situation. Iran will get the energy needs they have, however without raising an anxiety about any sensitive fuel cycle activities," he said.

The Islamic republic, just two weeks away from an IAEA deadline to prove it is not developing atomic weapons, on Thursday said it may bow to demands for tougher UN inspections.

During a lightning visit here Thursday by ElBaradei, Iran also promised to accelerate cooperation with the UN nuclear watchdog. However the IAEA chief added that Iran had still not agreed to stop enriching uranium -- one of the agency's key demands.

While denying developing nuclear weapons, Iran has maintained that it is being treated unfairly given that NPT signatories are obliged to provide mutual help with civil nuclear technology. The Islamic republic is subject to harsh US sanctions, and only Russia is helping it build its first nuclear power station.

Washington accuses Iran of trying to develop nuclear arms and has branded it part of an "axis of evil", along with Saddam Hussein's Iraq which it says held weapons of mass destruction, and North Korea, which has said it is developing atomic bombs.

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