"The director general (Rafael Grossi) is seriously concerned that there has still been no progress in clarifying and resolving the outstanding safeguards issues," the International Atomic Energy Agency (IAEA) said in a report seen by AFP.

Senior agency officials will conduct a technical visit to Tehran before the end of November, the report added.

"The agency has reiterated that at this meeting, it expects to start receiving from Iran technically credible explanations on these issues, including access to locations and material, as well as taking the samples as appropriate," it added.

The director general "reiterates that these issues... need to be resolved for the agency to be in a position to provide assurance that Iran's nuclear programme is exclusively peaceful," it added.

The UN watchdog has been pressing Iran to give answers on the presence of nuclear material at three undeclared sites, a key sticking point that led to a resolution criticising Iran being passed at a June meeting of the IAEA's board of governors.

In a separate report seen by AFP, the IAEA said Iran's stockpile of enriched uranium stood at 3,673.7 kilogrammes as of October 22, a decrease of 267.2 kilogrammes from the last quarterly report.

Iran has been enriching uranium well over the limits laid down in a landmark 2015 deal with world powers, which started to unravel when the United States withdrew from it in 2018.

On-off talks between Tehran and world powers have been underway since April last year to revive the deal, which gave Iran much-needed sanctions relief in return for curbs on its atomic programme.

US Secretary of State Antony Blinken late last month reiterated that he saw little scope to restore the deal, pointing to the clerical leadership's conditions, as major protests roil the country.