UNITED NATIONS - International Atomic Energy Agency (IAEA) Director General Rafael Grossi warned on Sunday that the international nuclear non-proliferation regime could crumble unless diplomacy returns, following Saturday's US strikes on Iranian nuclear sites.

"The nuclear non-proliferation regime that has underpinned international security for more than half a century is on the line. The dramatic events in Iran have become even more serious with last night's bombardments and the potential widening of the conflict," Grossi told an emergency meeting of the Security Council.

"We have a window of opportunity to return to dialogue and diplomacy. If that window closes, violence and destruction could reach unthinkable levels, and the global non-proliferation regime as we know it could crumble and fall," he warned.

Iran, Israel and the Middle East need peace and there is a path for diplomacy, he said, stressing the need to return to the negotiating table.

On the destruction of the US strikes, Grossi said there was damage to structures but no radioactive releases.

Craters are visible at the Fordow site, Iran's main location for enriching uranium to 60 percent, indicating the use by the United States of ground-penetrating munitions. At this time, no one, including the IAEA, is in a position to assess the underground damage at Fordow, he said.

At the Isfahan nuclear site, additional buildings were hit, with the United States confirming their use of cruise missiles. Affected buildings include some related to the uranium conversion process. Also at this site, entrances to tunnels used for the storage of enriched material appear to have been hit, he said.

At the Natanz enrichment site, the Fuel Enrichment Plant has been hit again, with the United States confirming that it used ground-penetrating munitions, he said.

Iran has informed the IAEA there has been no increase in off-site radiation levels at the three sites, said Grossi.

He said his agency has consistently underlined that armed attacks on nuclear facilities should never take place and could result in radioactive releases with grave consequences within and beyond the boundaries of the state that has been attacked. He again called for maximum restraint.

Military escalation threatens lives and delays a diplomatic solution for the long-term assurance that Iran does not acquire a nuclear weapon. It also threatens the global non-proliferation regime, he said.

"Let us not allow the window to close on diplomacy. Let us not allow the non-proliferation regime to fail. Irrespective of individual positions and views, one thing is certain, and this is the simple truth: we will not be safer if there are more nuclear weapons in more states around the world," said Grossi.