The battle had caused "unprecedented danger of a nuclear accident" and should not be repeated under any circumstances, said International Atomic Energy Agency (IAEA) director-general Rafael Grossi.

Grossi offered on Friday to travel to Chernobyl to negotiate with both sides to ensure the security of Ukraine's nuclear sites.

He was speaking hours after Russian forces had seized control of the Zaporizhzhia nuclear power plant following the battle with Ukrainian troops that caused a fire and fears of an accident.

Already on February 24, Russian troops invading Ukraine had taken control of the Chernobyl plant which, following the worst nuclear accident in history in 1986, is now encased in a giant sarcophagus.

"We should not be losing time," Grossi told reporters after opening the regular board meeting of his Vienna-based organization.

"We need to have an agreed clear framework of what is supposed to be done. So I hope that my consultations in the next few hours are going to be successful."

Grossi said Russia had informed him they preferred to meet somewhere other than Chernobyl.

"This is going to be part of a process of consultation," he said.

"There is safe operation but there are many, many questions on the ability to sustain this for much longer if we don't support this in some way."

Earlier Monday, Grossi said in his opening statement to the quarterly board meeting that a projectile hitting a building at Zaporizhzhia and causing a fire last week had been "a close call".

He told IAEA members: "The military operations at nuclear power facilities of Ukraine have caused unprecedented danger of a nuclear accident... Such a situation must not, under any circumstances, be repeated."

Expressing "deep concern", the IAEA said on Sunday that Ukraine had informed it that communication with the Zaporizhzhia plant had been disrupted and that its management was now under orders from the commander of the Russian forces.

On Sunday, Russian President Vladimir Putin assured French President Emmanuel Macron of the "physical and nuclear safety" of the Zaporizhzhia nuclear plant, according to the Kremlin.

Ukraine has four active nuclear power plants, providing about half the country's electricity, as well as stores of nuclear waste such as the one at Chernobyl.