The Russian-held Zaporizhzhia nuclear plant briefly suffered a complete disconnection from Ukraine’s power grid Thursday for the first time in its history, officials said, leading to large-scale power outages in nearly all the Russia-occupied cities of southern Ukraine.

Intensifying fighting around the plant, which is Europe’s largest, has sparked growing fears of a nuclear catastrophe. The two sides have traded blame for the attacks, while world leaders have called for a demilitarized zone around the site — a proposal Moscow has rejected.

The Zaporizhzhia Nuclear Power Plant — which provides one-fifth of Ukraine’s electricity and has been occupied by Russian forces for months — was still able to generate power to meet its own needs and keep essential systems working safely, according to Telegram posts from the Ukrainian nuclear energy agency, Energoatom.

The lines connecting the plant to infrastructure that helps transmit the power it generates to more than four million homes have come under repeated assault in recent weeks.

Ukrainian nuclear officials had recently warned that Russia was planning to disconnect the plant in a potentially risky effort to divert it to the Russian grid, the Guardian reported Wednesday.

On Thursday, the last two working reactors of the six at the southern Ukraine site were briefly disconnected after nearby fires sparked by shelling, Ukraine’s national nuclear company, Energoatom, said in its statement.

“As a result, the station’s two working power units were disconnected from the network,” it said, blaming Russian forces for the incident. “Thus, the actions of the invaders caused a complete disconnection of the ZNPP from the power grid — the first in the history of the plant.”

The fires broke out in the ash pits of a coal plant close to the nuclear plant, the energy company said. The nuclear plant’s security systems were functioning normally and engineers races to reconnect the power blocks to the grid, it added.

Security systems at the plant were activated after the power failure, Yevgeny Balitsky, the Russian-installed local government head, told the Russian RIA Novosti newswire — though he blamed Ukrainian troops for the shelling.  

“The station’s security systems were activated at the NPP,” he said “Reconnection was performed. Emergency services immediately went to the place. Specialists promptly carried out work to restore the power supply.”

The immediate impact of the fighting around the plant is being felt by hundreds of thousands of Ukrainians living in Russian occupied towns and cities across southern Ukraine, all of whom were plunged into blackouts when the disconnection occurred.

Dmytro Orlov, the exiled mayor of Enerhodar, which hosts the Zaporizhzhia plant, said the city was “once again on the verge of a humanitarian disaster.”

“Today, as a result of enemy shelling, the city was completely left without electricity and water,” he said on his Telegram channel.

Several hours later, he said that workers at the nuclear power plant managed to restore power to the city. But outages in other southern cities remained, including in the Russia-controlled port city of Berdyansk, more than 200 kilometers to the south.

The United Nations has offered to help facilitate a visit by International Atomic Energy Agency inspectors, but Moscow has insisted the inspectors travel to the plant via Russian-controlled territory rather than Ukraine proper.

IAEA head, Rafael Mariano Grossi, said that an agreement on a visit could be reached within days.

“We are very, very close to that,” Grossi told the France 24 news network, although he emphasized that the terms of the visit, and security arrangements to allow the inspectors to travel through a zone of active fighting, were yet to be finalized.