The plant has been at the centre of fighting since it was captured by Russian forces last year, and both sides have accused each other of compromising its safety.

"Due to the complete blackout, the nuclear power plant switched to powering its own needs from 20 diesel generators," Ukraine's nuclear energy operator said in a statement.

It said the plant was on "the verge of a nuclear and radiation accident" before Ukrainian specialists were able to promptly restore off-site power.

AFP was not able to immediately verify Ukraine's version of events.

The plant's Russian-installed operator confirmed it resorted to diesel generators overnight, but said that it had operated within safe limits and that no safety violations were reported.

The incident marks the eighth time the plant has been cut off from external power since the conflict began last year, the International Atomic Energy Agency (IAEA) warned.

"The most recent external power outage is yet another reminder about the precarious nuclear safety and security situation at the plant, which can be affected by events far away from the site itself," IAEA chief Rafael Grossi said.

"The IAEA continues to do everything it can to help prevent a nuclear accident. I also call on all parties not to take any action that could further endanger the plant," he added.

Since last year IAEA officials have been on the ground monitoring safety at the plant, which requires constant maintenance to prevent overheating.

It stopped supplying electricity to Ukraine's grid in September 2022, and has been repeatedly rocked by shelling and drone attacks throughout the 21-month conflict.