VIENNA - The extent of damage to Iran's Fordow uranium enrichment facility by US aerial attacks was not immediately possible to assess, the head of the United Nations nuclear watchdog said on Sunday.

Rafael Grossi, director general of the International Atomic Energy Agency (IAEA), confirmed in a statement that the Iranian nuclear sites of Fordow, Natanz, and Esfahan have been hit.

"It is clear that Fordow was also directly impacted, but the degree of damage inside the uranium enrichment halls can't be determined with certainty," he said, adding that the IAEA has been informed by the Iranian regulatory authorities that there has been no increase in off-site radiation levels after the attacks.

According to the IAEA analysis, the Natanz site, which has previously been extensively damaged, was again attacked during the strikes last night with ground-penetrating munitions; at the sprawling Esfahan site, which had already been struck several times by Israel, there has been extensive additional damage.

The IAEA had previously reported that several buildings at the Esfahan complex were damaged, some of which may have contained nuclear material.

"The latest attacks early this morning damaged other buildings in Esfahan. In addition, we have established that entrances to underground tunnels at the site were impacted," Grossi said.

He stressed the paramount importance of the countries involved urgently embarking on a diplomatic path aimed at ending the hostilities, saying this would also enable the IAEA to resume its crucial verification activities in Iran.

"We need to try to go back to the negotiating table as soon as possible. We have to allow the IAEA inspectors to return. The IAEA is ready to play its indispensable role in this process. We have been talking to Iran, we have been talking to the United States. We have to work for peace," Grossi said.