UN nuclear watchdog chief Rafael Grossi (Photo: AFP/VNA)

Grossi and Tehran agreed in March on a three-month plan to get to the bottom of the issue, which has been a source of tension between Iran and Western powers even during wider negotiations aimed at bringing Tehran and Washington back into full compliance with the 2015 Iran nuclear deal.

Grossi is due to report to the IAEA's Board of Governors on how talks on the open issues have progressed by the time the Board starts a quarterly meeting on June 6.