In a report seen by AFP, the International Atomic Energy Agency (IAEA) said it was "not in a position to provide assurance that Iran's nuclear programme is exclusively peaceful".

It said IAEA Director General Rafael Grossi was "increasingly concerned that Iran has not engaged with the Agency on the outstanding safeguards issues during this reporting period and, therefore, that there has been no progress towards resolving them".

The IAEA has been pressing Iran for answers on the presence of nuclear material at three undeclared sites.

Tehran, which maintains that its nuclear programme is exclusively peaceful, is insisting that the IAEA probe be concluded in order to revive the 2015 deal on its nuclear programme with world powers.

In a separate report also issued on Wednesday, the IAEA also said Iran was continuing to enrich uranium well over the limits laid down in the ailing 2015 deal, with its stockpile now over 19 times the limit set out in the accord.

The report said Iran's stockpile as of August 21 stood at an estimated 3,940 kilograms, up 131.6 kilograms from the last quarterly report.