The US bombing likely caused “very significant damage” to the underground parts of Iran’s Fordow nuclear site buried deep in a mountain, according to the head of the International Atomic Energy Agency (IAEA), the United Nations’ nuclear watchdog.

“Given the explosive payload utilized, and the extreme vibration-sensitive nature of centrifuges, very significant damage is expected to have occurred,” Rafael Grossi told the IAEA’s board of governors on Monday.

However, he cautioned that no group, including the IAEA, is currently able to fully assess the underground damage at Fordow.

The US struck the site early Sunday morning, using six B-2 bombers to drop 12 “bunker-buster” bombs on Fordow, each weighing around 30,000 pounds.