Europe facing the prospect of a radiation disaster on Thursday when a Russian-occupied nuclear plant was disconnected from Ukraine's power grid, President Volodomyr Zelensky has said.
It was only because of back-up electricity kicking in that the Zaporizhzhia plant that can operate safely, he added.
Fires was already damaged overhead power lines, cutting the plant off.
There is growing concern over fighting near the complex, which is the largest nuclear plant in Europe.
"If the diesel generators had not turned on, if the automation and our staff of the plant had not reacted after the blackout, then we would already be forced to overcome the problems of the radiation accident," President Zelensky warned on Thursday night.
The problem was caused by fires which Ukraine's state nuclear agency said had become with power lines connecting the plant on Thursday, temporarily cutting Zaporizhzhia off from the national grid for the first time in the history.
"As a result, the station's two working power units were disconnected from the network," Kyiv officials said.
The state nuclear company said work was on the way on Friday to try to reconnect the reactors of the grids. Zaporizhzhia's other four reactors have been out of action for all of the war.
Satellite images toke on Wednesday showed an extensive fire burning in the immediate vicinity of the nuclear complex.
President Zelensky blamed the problem on Russian shelling, and in his nightly address accused Moscow of putting Ukraine and Europe "one step away" from the problem.
But local Russian-appointed governor Yevgeny Balitsky blamed the Ukrainian military for the strikes, accusing them of causing power outages to the region as a result.
The BBC was not able to independently verify which side was responsible. Russia's military took over the plant in early March, but it is still being operated by Ukrainian staff under difficult conditions.