KYIV, Ukraine >> Europe’s largest nuclear power plant was relying on emergency diesel generators to run its safety systems Thursday after external power from the Ukrainian electric grid was again cut off, Ukrainian and U.N. officials reported.

Fighting in Ukraine has repeatedly damaged power lines and electrical substations that the Zaporizhzhia Nuclear Power Plant requires to operate its safety systems, forcing workers to turn to backup generators to cool its six reactors until regular power is restored. All six reactors have been shut down. The generators have enough fuel to maintain the plant in southeastern Ukraine for just 15 days, state nuclear power company Energoatom said.

“The countdown has begun,” Energoatom said, noting it had limited possibilities to “maintain the ZNPP in a safe mode.”

The U.N.’s International Atomic Energy Agency confirmed the switch to backup diesel generators and said that underlines “the extremely precarious nuclear safety situation.”

Rafael Grossi, the U.N. nuclear watchdog head, said relying on diesel generators “is clearly not a sustainable way to operate a major nuclear facility.”

Russia and Ukraine have traded blame during the war for shelling at and around the plant. Energoatom said Thursday that Russian shelling knocked out the last two high voltage transmission lines feeding the Zaporizhzhia plant. Russia gave a different account, blaming Ukraine.

The Russian state-run news agency Tass quoted an official at Russia’s nuclear power operator, Rosenergoatom, as claiming that Ukraine had switched off the two power lines and denied that Russian shelling of power lines had caused the problems. He said the move deprived the city of Energodar, where plant’s workers live, of heating.

Energoatom claimed Russian officials are trying to connect the power station to Russia’s power grid so it could supply electricity to Crimea, which Russia annexed in 2014, and Ukraine’s Donbas region, another area Putin annexed.

The human toll from earlier battles became evident again Thursday, when Ukrainian officials said 868 bodies of civilians, including 24 children, were found in liberated areas of the Kharkiv, Donetsk and Kherson regions. National police official Oleksii Serhieiev also told reporters 34 torture sites were found after Russian troops retreated from those areas, as well as the Kyiv, Sumy and Chernihiv regions.

Elsewhere on the battlefront, Russia used drones, missiles and heavy artillery to hit Ukrainian cities, leaving six civilians dead and 16 wounded, according to the president’s office. Russian attacks on Ukrainian President Volodymyr Zelenskyy’s hometown of Kryvyi Rih left several districts without electricity or water.

Further east in the Donetsk region, battles continued for the towns of Bakhmut and Avdiivka.

In the northeast, three Russian missiles hit the city of Kharkiv.