The U.N. nuclear watchdog and Iran ended a three-hour meeting on Monday without announcing any new action to help allay concern about Tehran's atomic activities, leaving it unclear whether headway was achieved, Reuters reported.
Contrary to usual practice, neither side made any statement after the talks at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna, held ahead of a May 15 deadline for Iran to provide information about detonators that can be used to set off an atomic explosive device.
They also did not say when they would meet again. The lack of a clear outcome may disappoint Western diplomats, who want Iran to move much faster in addressing suspicions about past atomic bomb research. Tehran denies any such work.
The meeting took place a day before the Iran and six world powers start a new round of negotiations - also in the Austrian capital - on a broad diplomatic settlement of the decade-old nuclear dispute.
Under a transparency and cooperation pact agreed 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.
Diplomatic sources told Reuters on Friday that the IAEA was seeking further clarification from Iran about one of those steps, concerning fast-acting detonators that can have both military and civilian applications.
Iran says it has already implemented the seven steps - including access to two uranium sites - but the sources suggested the IAEA still wanted more information about the so-called Explosive Bridge Wire (EBW) 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 activities in the past relevant for the development of nuclear weapons.
Iran says allegations of such work are baseless, but has offered to help clear up the suspicions with the U.N. agency.
The diplomatic sources said Iran in late April provided an explanation about the detonators, which it says are for non-nuclear uses, and that the IAEA had posed follow-up questions.
They said the IAEA also wanted to agree with Iran new measures to be taken after May 15, hoping these would tackle other sensitive issues linked to what the agency calls the possible military dimensions to Iran's nuclear programme.
Iran's talks with the IAEA and with big powers are closely linked as both focus on fears that Iran may be covertly seeking the capability to develop nuclear weapons. Iran says its uranium enrichment programme is a peaceful energy project only.
Western diplomats say Iran must start engaging with the IAEA's investigation and that this is central to the success of the powers' talks with Tehran aimed at an accord by late July.
Iran wants an end to sanctions severely hurting its oil-reliant economy. After years of confrontational standoff with the West, the election last year of pragmatist Hassan Rouhani as Iran's president created a new atmosphere more conducive to settling disputes via diplomacy.
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.