The UN atomic agency said on Friday it held "very constructive" talks with Iran, in the latest potentially positive sign since the more moderate Hassan Rouhani became president.

Following a flurry of meetings at the UN General Assembly this week in New York, Herman Nackaerts, chief inspector of the International Atomic Energy Agency, said in Vienna that the two sides would come together again on October 28.

"We will start substantial discussions (on October 28) on the way forward to resolving all outstanding issues," Nackaerts told reporters. "The talks were very constructive."

Iran's new envoy to the IAEA, Reza Najafi, said both sides had had "constructive discussions on a variety of issues".

The IAEA regularly inspects Iran's nuclear activities and every quarter its reports outline a continued expansion in defiance of UN Security Council resolutions.

Western countries want the IAEA to keep a closer eye on Iran to better detect any attempt to "break out" and produce highly-enriched uranium for an atomic bomb.

But the main focus of Friday's talks was the IAEA's wish for Iran to address allegations that before 2003, and possibly since, it conducted research work into making an actual nuclear weapon.

The agency has failed in 10 meetings since early 2012 to press Iran to grant it access to personnel, sites and documents related to these activities, set out in a major November 2011 report by the IAEA.

The allegations were based in large part on information provided to the IAEA from spy agencies like the CIA and Israel's Mossad, intelligence which Iran rubbishes and complains it has not even been allowed to see.

The sites include the Parchin military base where the IAEA wants to probe claims that scientists conducted explosives tests that would be "strong indicators of possible nuclear weapon development".

Western countries have accused Iran of literally bulldozing evidence at Parchin, and IAEA head Yukiya Amano said in June that heavy construction work spotted by satellites means "it may no longer be possible to find anything even if we have access".

Jigsaw puzzle

Providing some hope that some progress might now be made is that under Rouhani, Iran has been sounding considerably more conciliatory than under his more hardline predecessor Mahmoud Ahmadinejad.

Friday's meeting was the first IAEA gathering involving Najafi, who arrived in Vienna earlier this month professing a "strong political will" to engage.

"We welcome the recent developments and statements made by Iran about their willingness to engage, to resolve the nuclear issue expeditiously," Nackaerts said as he went into the meeting.

On Thursday new Foreign Minister Mohammad Javad Zarif met with counterparts from the five permanent members of the UN Security Council plus Germany (the so-called P5+1) at the UN General Assembly, including US Secretary of State John Kerry.

Kerry said Zarif's presentation was "very different in tone, and very different in the vision that he held out with respect to the possibilities in the future".

Zarif said the talks agreed to "jumpstart" work on a deal and "move towards finalising it, hopefully, within a year's time".

Kerry shook hands and met briefly one-on-one with a smiling Zarif on the sidelines in one of the foes' highest-level encounters since the 1979 Islamic Revolution.

The six powers will meet again for talks with Iran on October 15 and 16 in Geneva, the EU's foreign policy chief and P5+1 chief negotiator Catherine Ashton said in New York.

This diplomatic track is separate from that of the IAEA, concentrating more on Iran's current activities, most notably uranium enrichment, with Tehran seeking an easing of painful UN and Western sanctions.

But a deal with the IAEA on probing claims of past weaponisation work is a key part of the jigsaw needed to finally peacefully resolve the standoff over Iran's nuclear work after a decade of trying.

"The area where Iran is most willing and able to make concessions is the area of transparency and that means more cooperation with the IAEA," said Mark Fitzpatrick from the International Institute for Strategic Studies.