Experts from the UN nuclear watchdog held talks in Tehran Monday in their investigation into decade-old allegations of possible military dimensions to Iran's nuclear programme, the ISNA news agency reported.
The International Atomic Energy Agency delegation's visit comes as the United States and other major world powers seek to conclude a comprehensive agreement on Iran's controversial nuclear activities.
While Iran is negotiating limits on its programme with world powers, the IAEA has been pressing leaders in Tehran for years to address allegations that prior to 2003, and possibly since, they conducted research into the technology required to build a nuclear weapon.
In 2011, the IAEA expressed concern about Iran's development of "exploding bridge wire detonators" because of their "possible application in a nuclear explosive device."
It is one of a number of allegations of past weapons research which Tehran denies but which the watchdog says it has yet to satisfactorily answer.
Another technical issue to be discussed during the visit concerns "modelling and calculations" on neutrons, the sub-atomic particles which trigger the fission of uranium, ISNA reported.
After some progress last year, the IAEA investigation stalled.
In its latest report on February 19, the IAEA said Iran had not provided "any explanation regarding the two outstanding practical issues".
But a spokesman for Iran's Atomic Energy Agency told state television on Sunday that "these two issues can be finalised during the visit of the IAEA delegation".
IAEA chief Yukiya Amano has urged Iran to provide answers quickly.