U.S. bombing probably caused "very significant" damage to the underground areas of Iran's Fordo 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 and eliminate sites including the Fordow uranium-enrichment plant dug into a mountain.

"At this time, no one, including the IAEA is in a position to have fully assessed the underground damage at Fordow," Grossi said in a statement to an emergency meeting of the International Atomic Energy Agency's 35-nation Board of Governors.

The IAEA has not been able to carry out inspections in Iran since Israel started its military strikes on nuclear facilities there on June 13.

"Given the explosive payload utilized and the extreme(ly) vibration-sensitive nature of centrifuges, very significant damage is expected to have occurred," Grossi added.

Beyond the level of damage done to Fordo's underground enrichment halls, one of the biggest open questions is the status of its stock of enriched uranium, particularly its more than 400 kg of uranium enriched to up to 60% purity, a short step from the roughly 90% that is weapons grade.

That is enough, if enriched further, for nine nuclear weapons, according to an IAEA yardstick, though Iran says its intentions are peaceful and it does not seek atom bombs.

Iran did, however, inform the IAEA on June 13 that it would take "special measures" to protect its nuclear materials and equipment that are under so-called IAEA safeguards, the oversight provided for by the nuclear Non-Proliferation Treaty, Grossi said.