The chief of the U.N. nuclear agency said Tuesday he expected to sign a breakthrough deal with Iran to boost cooperation on a probe into the country's nuclear activities.

Yukiya Amano, head of the International Atomic Energy Agency, spoke a day after meeting Iranian officials and a day ahead of a key nuclear meeting in Baghdad between Tehran and a group of leading world powers.

"[A] decision was made to conclude and sign the agreement. ... I can say it will be signed quite soon," Reuters quoted Amano as telling reporters at the Vienna airport after he returned from Tehran.

Amano's visit aimed at giving his team of inspectors' greater freedom to investigate Iran's suspected atomic bomb research.

He said the outcome of his meeting in Tehran was an "important development". "Some differences" between the IAEA delegation and Iran's top nuclear negotiator Saeed Jalili remained, but would not prevent an agreement, Amano said.

"We understand each other's position better," Amano added.

Amano said the issue of access to the Parchin military site, southwest of Tehran, would be addressed as part of the deal but failed to provide further details.

Access to the Parchin facility is one of the IAEA's delegation key demands. The nuclear agency believes that in 2003 Iran ran explosive tests needed to set off a nuclear charge inside a pressure chamber. Iran denies the accusation.

Attention will next shift to the meeting in Bagdad where talks between Iran and the United States, Russia, China, Britain, France and Germany will focus on the Islamic Republic's nuclear programme.

The six world powers will attempt to persuade Iran to curb its nuclear programme, while Iran is expected to seek prevent additional US and EU sanctions on its oil exports.

Iran has dismissed fears it will use its nuclear programme to build weapons, saying it will be used for peaceful aims only and insisting its reactors are for power and medical research.

But Western powers remain concerned that Iran's production of uranium enriched to 20 per cent could be further boosted to warhead-grade material. They now want Iran to halt uranium enrichment.