"I have repeatedly condemned any military escalation in the Middle East," the secretary-general told an emergency meeting of the UN Security Council.

"The people of the region cannot endure another cycle of destruction. And yet, we now risk descending into a rathole of retaliation after retaliation."

Rafael Grossi, director of the International Atomic Energy Agency (IAEA), also called for restraint as he voiced fears over "potential widening" of the conflict.

"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," Grossi said.

Speaking to the Security Council by video link, he said there were visible craters at Iran's key Fordo nuclear facility, "indicating the use by the United States of America of ground-penetrating munitions."

But Grossi noted that no one had been able to assess the underground damage at Fordo.

He added 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 which has been attacked."

On Sunday, Russia, China and Pakistan circulated a draft resolution with other Security Council members that calls for an "immediate ceasefire" in Iran.

The text, seen by AFP, also "condemns in the strongest terms the attacks against peaceful nuclear sites and facilities."

But the Council is very much divided, with other members such as France and Britain instead calling for Iran to exercise restraint.

Israel's UN ambassador, meanwhile, rejected the proposed resolution.

"The US and Israel do not deserve any condemnation, but rather an expression of appreciation and gratitude for making the world a safer place," Danny Danon said.

Amir Saeid Iravani, Iran's envoy to the UN, slammed the United States for having "once again resorted to illegal force."

Washington, he said, has "waged a war against my country, under a fabricated and absurd pretext: preventing Iran from acquiring nuclear weapons."