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 United States dropped the biggest conventional bombs in its arsenal on Iranian nuclear facilities on Sunday, using those bunker-busting munitions in combat for the first time to try to eliminate sites including the Fordow uranium-enrichment plant dug into a mountain.