Iran has agreed to clarify intelligence alleging that it studied how to design nuclear bombs, a gesture the UN nuclear watchdog chief today called a "milestone".
Tehran has previously denied the reports but declined to address them in detail.
"(This) is a certain milestone and hopefully by the end of May we'll be in position to get the explanation and clarification from Iran as to these alleged studies," said International Atomic Energy Agency Director Mohamed ElBaradei.
"This in my view is a positive step. I hope we will be able to clarify within the next few weeks this important issue," he said during a visit to the Bosnian capital Sarajevo.
Iran had rejected the reports as baseless, fabricated or irrelevant and said exchanges with IAEA officials on the issue had resolved it and there would be no more discussions. But the UN watchdog has insisted Iran back up its denials with proof.
"An agreement was reached during meetings in Tehran (on Monday and yesterday) on a process that aims to clarify the so-called alleged studies during the month of May," IAEA spokeswoman Melissa Fleming said in a statement in Vienna.
She did not elaborate. Iran gave an upbeat assessment yesterday of the two days of talks with Olli Heinonen, the IAEA's safeguards chief and top investigator, saying they were "positive" but not saying what was discussed.
Iran says its nuclear campaign is a peaceful quest for an alternative source of electricity so that it can export more of its oil and gas. It is the world's fourth-largest oil exporter.