IAEA chief Rafael Grossi also said up to now his agency had "no indication" that any of the nuclear installations have been damaged or hit in Iran.

The Vienna-based International Atomic Energy Agency (IAEA) held an extraordinary session on Iran on Monday at the request of Russia, a key ally of Tehran, following the same request by Iran over the weekend.

"I reiterate my call on all parties to exercise maximum restraint to avoid further escalation," Grossi said in his statement, released by the IAEA, as he opened the closed-door session.

"Iran and many other countries in the region that have been subjected to military attacks have operational nuclear power plants and nuclear research reactors, as well as associated fuel storage sites, increasing the threat to nuclear safety," he added.

"Let me underline that the situation today is very concerning. We cannot rule out a possible radiological release with serious consequences, including the necessity to evacuate areas as large or larger than major cities," he said.

The agency was trying to contact the Iranian nuclear regulatory authorities "with no response so far", he said.

"We hope this indispensable channel of communication can be reestablished as soon as possible," he said.

He called for diplomatic negotiations to resume "as quickly as possible".

Grossi attended two Oman-mediated rounds of talks between Iran and the US this month in Geneva on Iran's nuclear programme.

"An understanding eluded the parties this time. I am sure we are, quite understandably, feeling a strong sense of frustration," he said.

The extraordinary meeting preceded an already scheduled regular session of the IAEA's board of governors, which represents 35 countries.

Western countries led by the United States and Israel, Iran's arch-enemy, accuse the Islamic republic of seeking to acquire nuclear weapons.

Tehran denies having such military ambitions, but insists on its right to this technology for civilian purposes.