The UN nuclear watchdog declined to say whether Iran had met a deadline on Thursday for starting to address suspicions it may have carried out atomic bomb research, adding to signs of limited progress so far.
Under a cooperation pact agreed between the two sides in November, Iran was to implement seven transparency steps by May 15 to help allay international concern about its disputed nuclear program, which the West fears may have military ends.
On the most sensitive of those - for Iran to provide information about detonators that can, among other things, be used to set off an atomic explosive device - diplomats have said the UN atomic agency was seeking further clarification.
How Iran responds to questions about so-called Explosive Bridge Wire detonators is seen as a litmus test of its readiness to begin cooperating with a long-stonewalled investigation into what the UN agency calls the possible military dimensions (PMD) of the country's nuclear program.
Iran denies Western allegations that it has been seeking to develop the capability to make nuclear weapons but has offered to work with the UN International Atomic Energy Agency (IAEA) to resolve its concerns.
The IAEA-Iran talks are separate from those between Tehran and six world powers - the United States, France, Germany, Britain, China and Russia - aimed at reaching a broader deal to settle the decade-old nuclear dispute by late July.