Ukraine’s nuclear state operator said on Thursday that Russia has shelled and damaged power lines connecting Europe’s largest nuclear power plant to the Ukrainian grid, leaving the plant reliant on diesel generators again.

The generators have enough fuel to maintain the Zaporizhzhia nuclear power plant in southeastern Ukraine for just 15 days, Energoatom said in a post on its Telegram channel.

“The countdown has begun,” Energoatom said, noting it had limited possibilities to “maintain the ZNPP in a safe mode”, raising fears of a potential nuclear disaster.

With its six reactors inoperative, the plant relies on outside electricity to cool its spent fuel.

Russia and Ukraine have traded blame for months amid the war for shelling at and around the plant that the UN’s nuclear watchdog has warned could cause a radiation emergency.

The nuclear power plant lies within part of the Zaporizhzhia region that has been occupied by Russian forces since the early days of the war, which began when Russia invaded Ukraine on February 24. Although Russian President Vladimir Putin signed a decree transferring the plant to Russian ownership, Ukrainian workers continue to run the plant.