The two-day meeting between Iran's envoy to the International Atomic Energy Agency, Ali Asghar Soltanieh, and chief inspector Hermann Nackaerts aimed at clarifying suspicions Tehran had done nuclear weapons research.
"We are here to continue our dialogue with Iran in a positive spirit," Nackaerts told reporters as he went into the talks at Iran's Vienna embassy.
"The aim of our two days is to reach an agreement on an approach to resolve all outstanding issues with Iran. In particular the clarification of the possible military dimension remains our priority."
The last time the two met officially was in early February in the second of two visits to Tehran -- each branded a "failure" by Washington -- after which the IAEA said it had "major differences" with the Islamic republic.
The watchdog said Iran brushed aside extensive claims made in a November IAEA report that at least until 2003, and possibly since, activities took place which could only conceivably be aimed at developing nuclear weapons.
Iran also denied Nackaerts access to the Parchin military site near Tehran where, the November report alleged, Iran had conducted suspicious explosives tests in a large metal container, the IAEA said.
IAEA chief Yukiya Amano said recently that access to Parchin was a "priority", and Western diplomats said the site would be at the heart of this week's meeting in Vienna.
Inspectors already visited Parchin near Tehran twice in 2005 and found nothing, Iran says, but the IAEA says it has since obtained additional information that makes it want to go back for another look.
Western countries also suspect that Iran is cleaning up the site in order to remove evidence.
Soltanieh said in March that any allegations of "sanitization" of the site were "a childish (and) ridiculous story made out of nothing."
But since the IAEA visits, 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 -- Britain, China, France, Russia and the United States plus Germany -- held their first talks in 15 months in Istanbul in April, and agreed to more in-depth discussions in Baghdad on May 23.
The talks in Vienna could give early clues on whether the good atmosphere seen in Turkey will continue in Iraq, where the P5+1 want to get down to the nuts and bolts of the almost decade-old dispute over Iran's nuclear programme.
"The world powers will be watching closely to see if there are any signs of Iran shifting its position and becoming more accommodating," said Mark Fitzpatrick of the International Institute for Strategic Studies in London.
Any signs that Iran is using cooperation with the IAEA as a "bargaining chip" would however "get the Baghdad discussions off to a bad start," he told AFP.
Most worrying to the P5+1 is Iranian 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.
One way of easing concerns would be for Iran to submit to more intrusive IAEA inspections by implementing the "additional protocol" of the nuclear non-proliferation treaty.
Tehran says it wants international acceptance of its right to peaceful nuclear activities, for sanctions to be lifted and for the threat of US and Israeli military action to disappear.
Iran's top negotiator, Saeed Jalili, was quoted Sunday as saying that Tehran was "awaiting actions to secure Iranian people's trust" in Baghdad.