UN atomic watchdog chief Mohamed ElBaradei said on Wednesday that Iran has agreed to clarify claims it has studied how to design nuclear weapons by the end of May.
"It is my understanding that hopefully by the end of May we will be in a position to get an explanation and clarification from Iran as to these alleged studies," ElBaradei told reporters during a visit to Bosnia.
The head of the International Atomic Energy Agency said its deputy director general, Olli Heinonen, reported that he and Iranian officials "agreed on a process" to start explaining the studies within a few weeks.
Heinonen visited Tehran on Monday and Tuesday to press Iran over claims it has carried out so-called "weaponisation studies," according to the Vienna-based IAEA.
"This in my view is a positive step. I hope they will be able to clarify witnin a few weeks this important issue," ElBaradei said.
Iran insists its nuclear programme is entirely peaceful and aimed solely at generating energy.
Its refusal to suspend sensitive uranium enrichment operations -- which the West fears could be used to make a nuclear weapon -- has already led to three sets of UN Security Council sanctions against Tehran.
Despite more than four years of intensive investigation, the IAEA has never been able to confirm the nuclear drive is peaceful and bring its probe to a conclusion.