Senior officials at the International Atomic Energy Agency, IAEA, have ended two days of talks with Iranian officials over allegations that Tehran may have carried out tests on triggers for atomic weapons, the Fars news agency reports.
Thursday's report went on to say that the two sides agreed to another round of negotiations scheduled for February 12.
A senior IAEA diplomat, demanding anonymity because he is not authorised to speak on the matter, said that the two-day talks were "not going very well", shortly before they came to an end.
Herman Nackaerts, head of the UN team, had hoped the IAEA would be able to "finalise the structured approach'' that would outline what the agency can and cannot do in its investigation.
The IAEA, whose mission is to prevent the spread of nuclear weapons, has been trying for a year to negotiate a so-called structured approach with Iran giving the inspectors access to sites, officials and documents for their long-stalled inquiry.
World powers were monitoring the IAEA-Iran talks for any signs as to whether Tehran, facing intensifying sanctions pressure, may be prepared to finally start tackling mounting international concerns about its nuclear activity.
The six powers - the United States, France, Germany, China, Russia and Britain - and Iran may resume their separate negotiations later in January to try to reach a broader diplomatic settlement. They last met in June.