The U.N. nuclear watchdog and Iran ended a three-hour meeting on Monday without announcing any new action to allay concerns about Tehran's atomic activities, leaving it unclear whether headway was achieved.
The International Atomic Energy Agency (IAEA) indicated after the talks that some more work was still needed for the full implementation of a series of nuclear transparency steps that Iran had agreed to take by May 15. It did not give details.
Reuters last week cited diplomatic sources as saying the IAEA is seeking further clarification from Iran about one of those measures - information about fast-acting detonators that have civilian and military uses, including setting off an atomic explosive device.
The meeting took place a day before the Iran and six world powers start a new round of negotiations in Vienna on a broad diplomatic settlement of the decade-old nuclear dispute.
Under a transparency and cooperation agreement reached with the IAEA in November, Iran was to take seven measures by May 15 in a phased process to shed more light on a nuclear programme the West fears may be aimed at developing atomic bomb capability.
The IAEA statement said the two sides had reviewed progress on implementation of the steps.
"The agency noted that Iran has taken several actions and that some related work continues," it said, without elaborating.
The lack of a clear outcome in Monday's meeting may disappoint Western diplomats, who want Iran to move much faster in addressing suspicions about past atomic bomb research. Tehran denies any such work.
Iran says it has already implemented the seven steps - including access to two uranium sites - but the sources suggested the IAEA still wanted more clarification about the so-called Explosive Bridge Wire detonators.
How Iran responds to questions about its development of this type of equipment is seen as an important test of its willingness to cooperate fully with a long-stalled IAEA investigation into suspected past activities relevant to the development of nuclear weapons.
Iran says allegations of such work are baseless and has offered to help clear up the suspicions.
The diplomatic sources also said the IAEA wants to agree with Iran on new measures to be taken after May 15, to tackle other sensitive issues linked to what the agency calls the possible military dimensions of Iran's nuclear programme.
"Discussions on additional practical measures to be implemented in the next steps are ongoing," the IAEA said in the
statement, suggesting no agreement had yet been reached.
Western diplomats say Iran must start engaging with the IAEA's investigation and that this is central to the success of the separate talks between the six powers and Tehran aimed at an accord by late July.
But diplomats say Iran and the powers - the United States, France, Germany, Russia, China and Britain - remain far apart on what a long-term deal to resolve the dispute, and dispel fears of a new Middle East war, would look like.
Editing by Mark Heinrich and Robin Pomeroy