One day after the May 19 helicopter crash which killed Raisi and others "Iran indicated that due to the 'special circumstances', it was no longer appropriate to hold substantive discussions" and a new date would be set, according to a confidential report seen by AFP on Monday.

Tensions between Iran and the IAEA have repeatedly flared since a 2015 deal curbing Tehran's nuclear programme in exchange for sanction relief fell apart.

In recent years, Tehran has decreased its cooperation with the IAEA by deactivating surveillance devices needed to monitor the nuclear programme and barred UN inspectors.

Earlier this month, IAEA chief Rafael Grossi visited Iran in a bid to improve cooperation with Tehran.

After returning from his trip, Grossi decried "completely unsatisfactory" cooperation.

In a separate confidential report seen by AFP ahead of an IAEA board of governors' meeting next week, the agency said Iran's estimated stockpile of enriched uranium had reached more than 30 times the limit set out in the 2015 accord between Tehran and world powers.

According to the report, Iran's total enriched uranium stockpile was estimated at 6,201.3 kilogrammes as of May 11, up by 675.8 kilogrammes from the last quarterly report in February.

EU-mediated efforts to revive the deal -- bringing the US back on board and Iran back into compliance -- have so far been fruitless.