Ukraine’s largest nuclear power plant was cut off from the country’s electricity grid, setting off a mass power outage in the adjacent area after fires damaged its last functioning transmission line, Ukraine’s nuclear power company said Thursday.

Emergency backup systems kicked in and helped sustain crucial operations, but the incident heightened fears of a disaster at the Zaporizhzhia nuclear power plant (ZNPP), which is also the largest atomic energy plant in Europe and is located in an area occupied by invading Russian forces.

Fighting in the vicinity of the plant has led to acute worries of a potential catastrophe and to urgent calls from many world leaders for U.N. nuclear experts to be allowed to visit the site.

Russian and Ukrainian officials traded blame for shelling at the plant, which they said had resulted in the disconnection from the power grid — the first time it has ever been cut off.

See omnystudio.com/listener for privacy information.