The head of the United Nations nuclear watchdog said on Monday that “very heavy damage” is expected at Iran’s underground facility at Fordo after a U.S. airstrike there this weekend with sophisticated bunker-buster bombs.

U.S. bombing probably caused "very significant" damage to the underground areas of Iran's Fordow uranium enrichment plant dug into a mountain, though no one can yet tell the extent, U.N. nuclear watchdog chief Rafael Grossi said on Monday.

The International Atomic Energy Agency reported Monday that Iran’s Fordo nuclear facility likely suffered extensive underground damage from recent US airstrikes, due to the sensitive nature of its centrifuge infrastructure and the powerful explosives used in the attack.