BERLIN (AP) — 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 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.

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 this month that the situation was like “flying in a heavily clouded sky.”

Grossi 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.