Talks between the UN atomic agency and Iran saw a "good exchange of views" and will resume on Monday, two days before world powers meet Tehran representatives in Baghdad, the watchdog's chief inspector said.
"During these two days we discussed a number of options to take the agency verification process forward in a structured way," Hermann Nackaerts said Tuesday in a joint statement to journalists with Iran's ambassador Ali Asghar Soltanieh.
"We had a good exchange of views and we will meet again on Monday," he said.
Soltanieh said: "We had fruitful discussions in a very conducive environment. We have made progress on this issue regarding preparing and negotiating the modality framework for resolving our outstanding issues."
Neither gave further details, including on whether International Atomic Energy Agency access to the Parchin military site was discussed at the talks at Iran's embassy here.
A day earlier Nackaerts had said he wanted to press Iran for "access to people, documents, information and sites" in its contested nuclear programme.
In particular Nackaerts wants Iran to address claims made in an extensive IAEA report in November that at least until 2003, and possibly since, there were "activities relevant to the development of a nuclear explosive device."
He also wants access to the Parchin military base near Tehran where the IAEA report said Iran had conducted explosives tests in a metal container.
In two visits to Tehran in January and February, branded a "failure" by Washington, the IAEA said Iran denied Nackaerts access to Parchin and dismissed out of hand the claims made in the report as based on forgeries.
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Western countries accuse Iran of removing evidence at the base, but Soltanieh said in March that any allegations of "sanitization" of the site were "a childish (and) ridiculous story made out of nothing."
Iran, which disputed the IAEA's version of events during the two visits, says that since no nuclear activities took place at Parchin, it is under no obligation to allow the IAEA in.
It has said it would allow access to Parchin but only as part of a broader agreement on "modalities" for future cooperation.
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.
Parchin is in fact "believed to be less significant than other sites on the IAEA's wish list," said Mark Hibbs, analyst at the Carnegie Endowment for International Peace and an expert in nuclear proliferation.
But the agency chose the site before others because its information on Parchin, unlike on other locations, was not based on information from foreign intelligence services -- sources which the IAEA refuses to reveal.
Hibbs said that "some IAEA officials see Tehran's refusal of access as a challenge to the IAEA's primacy in setting the agenda for inspections, and for that reason the IAEA will continue to request access to that site as a matter of principle."
The Vienna talks were being closely watched for signs of whether the Iranians might make substantial concessions on May 23 in their talks in Baghdad with P5+1 powers -- the United States, Russia, China, Britain, France and Germany.
Greater cooperation with the IAEA is a key area where the P5+1 would like to see the Iranians becoming more accommodating in order to better verify Tehran's assertion that its nuclear programme is entirely peaceful.
The IAEA has been saying for several years now that it is unable to confirm this, leading to several rounds of UN Security Council resolutions calling on Iran to halt all enrichment of uranium.
Enriched uranium can be used for peaceful purposes such as power generation and producing medical isotopes, but also when highly purified as the core of an atomic bomb.
Other Iranian concessions might include suspending the enrichment of uranium to purities of 20 percent. In return they want sanctions to be eased, but there is little in the way of relief the P5+1 can offer at this point, experts say.