The United Nations' atomic watchdog says it believes Iran has further increased its stockpile of highly enriched uranium in breach of a 2015 accord with world powers.

The International Atomic Energy Agency (IAEA) told member nations in its confidential quarterly report Wednesday that Iran has an estimated stock of 17.7 kilograms (39 pounds) of uranium enriched to up to 60% fissile purity, an increase of almost 8 kilograms since August. The total amount now includes 113.8 kilograms enriched to 20%, up from 84.3 in September, the report said.

Such highly enriched uranium can be easily refined to make atomic weapons, which is why world powers have sought to contain Tehran’s nuclear program.

The Vienna-based agency told members that it is still not able to verify Iran’s exact stockpile of enriched uranium due to the limitations Tehran imposed on U.N. inspectors earlier this year.

The IAEA has been unable to access surveillance footage of Iranian nuclear sites or of online enrichment monitors and electronic seals since February.

The agency's chief, Rafael Mariano Grossi, told The Associated Press (AP) this month that the situation was like "flying in a heavily clouded sky.” Grossi, who is expected to travel to Tehran this month for direct talks with Iranian officials on restoring the agency's ability to know in real-time what the country is doing, had expressed his concern earlier on Nov. 12. He said he had hoped to meet Iranian officials ahead of the next meeting of the IAEA's Board of Governors, which was scheduled for November.

The head of the agency "will arrive on the evening of Monday, Nov. 22 in Tehran," Iran's atomic agency spokesperson told Fars news agency Wednesday. Grossi will meet Foreign Minister Hossein Amir-Abdollahian and the head of Iran's atomic agency, Mohamed Eslami, the spokesperson added.

Grossi's last visit to Tehran was in September, when he said he had "technical discussions" with Eslami. He clinched a deal on access to surveillance equipment at Iran's nuclear facilities but had hoped to return to the country soon for more detailed discussions.

Grossi's visit comes ahead of the resumption on Nov. 29 of nuclear talks in Vienna, stalled since June. The talks aim to restore a 2015 deal that offered Tehran relief from sanctions in exchange for major curbs on its nuclear activities.

The U.S. unilaterally pulled out of the deal in 2018 under the administration of president Donald Trump, but talks to revive it began earlier this year. President Ebrahim Raisi said Iran is "absolutely serious" about the nuclear talks, in a phone call Tuesday with his Russian counterpart Vladimir Putin. The Vienna talks will be attended by the remaining parties to the deal – Britain, China, France, Germany and Russia – while the U.S. will participate in negotiations indirectly.

On Wednesday, IAEA also "categorically" denied its cameras played a part in a June attack on an Iranian nuclear facility, after Tehran said it was investigating the possibility. The IAEA "Director General categorically rejects the idea that Agency cameras played a role in assisting any third party to launch an attack on the TESA Karaj complex," said a report by IAEA seen by Agence France-Presse (AFP).

Iran has told the IAEA that "its 'security and judicial authorities' were 'investigating whether the terrorists have used the Agency cameras to launch an attack on the complex'," the report said, referring to a building near Karaj about 50 kilometers (30 miles) from the capital Tehran.