World powers will on Monday be closely following Iran's first talks with the UN nuclear agency in three months, for clues on whether Tehran means business at an upcoming crunch meeting in Baghdad.

The last time that Iran's envoy to the International Atomic Energy Agency, Ali Asghar Soltanieh, and chief inspector Hermann Nackaerts met officially was in early February on the second of two fruitless IAEA visits to Tehran.

Soltanieh said the resumption of the talks "proves Iran's determination to cooperate with the agency, confirms that claims against Iran are baseless, and shows that all of the Islamic republic's nuclear activities are peaceful."

The two days of discussions behind closed doors in Vienna "will be a good test of Iran's intentions in the whole (nuclear) issue," Bruno Tertrais, senior research fellow at the Foundation for Strategic Research in Paris, told AFP.

"The world powers will be watching closely to see if there are any signs of Iran shifting its position and becoming more accommodating," agreed Mark Fitzpatrick of the International Institute for Strategic Studies in London.

He cautioned however that any signs that Iran was using cooperation with the IAEA as a "bargaining chip" ahead would "get the Baghdad discussions off to a bad start".

The IAEA said after the latest visit -- branded a "failure" by Washington -- that "major differences" existed with Tehran on how to ease suspicions that Iran's nuclear programme is not, as it claims, purely peaceful.

In particular, the agency said that Iran had refused Nackaerts access to the Parchin military site near Tehran where a major IAEA report in November alleged Iran had conducted suspicious explosives tests in a large metal container.

That extensive report focused on a number of areas where the IAEA believes Iran carried out activities the agency said could only conceivably be aimed at developing nuclear weapons, at least until 2003 and possibly since.

During the visits, Iranian officials stuck doggedly to their assertion that the report, which has prompted Western countries to ramp up sanctions and raised speculation of Israeli plans for air strikes, was based on forgeries.

But since February, hopes have emerged that with new US and EU sanctions due to bite from mid-2012, Iran's approach has changed.

Iran and the P5+1 world powers -- Britain, China, France, Russia and the United States plus Germany -- held their first talks in 15 months in Istanbul in April. They were hailed by EU foreign policy chief Catherine Ashton as "constructive and useful".

Agreeing to meet again in May, the White House applauded what it called the Iranians' "positive attitude", while Iran's envoy Saeed Jalili praised the "desire of the other side for dialogue and cooperation".

Monday and Tuesday's talks in Vienna could give early clues on whether these good vibrations will continue in Baghdad on May 23, when the P5+1 want to get down to the nuts and bolts of the almost decade-old dispute over Iran's nuclear programme.

In particular, they want Iran to suspend the enrichment of uranium to purities of 20 percent, most notably at the Fordo site inside a mountain near the holy city of Qom. Uranium enriched to 90 percent can be used in a nuclear bomb.

They also want Iran to submit to more intrusive IAEA inspections by implementing the "additional protocol" that as a signatory of the nuclear non-proliferation treaty (NPT) Tehran briefly adhered to but dropped in 2006.

Tehran's aims are international acceptance of its right to peaceful nuclear activities, for sanctions to be lifted and for the threat of Israeli military action to disappear. It also wants to be reassured, after decades of mutual mistrust, that the West is not seeking regime change.

But vital in building confidence will be Iran addressing at least some of the IAEA's evidence of weaponisation, David Albright from the Institute for Science and International Security and former IAEA inspector Olli Heinonen said in a joint report.

"If Iran does not start providing clarifications about the military dimension of its efforts, it risks poisoning the negotiations with the P5+1 and makes military strikes that much more likely," they warned.