Ukraine said Saturday that two power lines connecting its electricity grid to the Zaporizhzhia nuclear plant were cut overnight, putting the plant at risk of an "accident".
The plant has been in 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 off-site power was restored by Kyiv.
"The situation was actually saved by the prompt actions of Ukrainian specialists who restored power supply to the plant from the Ukrainian power grid," it said.
AFP was not able to immediately verify Ukraine's version of events.
The International Atomic Energy Agency confirmed Saturday that the plant temporarily lost off-site power overnight, saying it was the eighth such blackout since the conflict began.
It said it would provide further details later.
IAEA officials have been monitoring safety at the plant, which requires constant maintenance to prevent overheating.
The six-reactor plant at Zaporizhzhia has been rocked by repeated shelling and drone attacks throughout the 21-month conflict, and stopped supplying electricity to the Ukraine's grid in September 2022.