Ukraine has warned that Russia was planning to disconnect the plant in a potentially risky effort to divert it to the Russian grid.On Thursday, the last two working power units at the site in southern Ukraine were disconnected after nearby fires sparked by shelling, Ukraine's national energy company, Energoatom, said in a statement posted to Telegram. 

    "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 work was underway to reconnect one of the power blocks to the grid, it added.

    Security systems at the plant were activated after the power failure, a local official said, according to the Russian state news agency RIA Novosti.

    "The station’s security systems were activated at the NPP. Reconnection was performed. Emergency services immediately went to the place. Specialists promptly carried out work to restore the power supply," Alexander Volga, head of the Russian-installed regional administration, said according to the news agency. Local officials blamed shelling by Ukrainian troops for the fires, RIA reported.

    NBC News has not been able to verify the claims of either side. 

    The disruption at the nuclear complex unfolded as the city of Enerhodar, where the plant is based, suffered a power supply failure.

    "Enerhodar once again found itself on the verge of a humanitarian disaster," Mayor Dmytro Orlov said in a statement on Telegram.

    "Today, as a result of enemy shelling, the city was left completely without electricity and water," he said. "Now we are finding out how critical the situation is and whether it can be corrected in the near future."

    In a later update, Orlov said restoration of the power supply had begun "thanks to the employees of utility companies and specialists of the (power plant)." 

    "After the completion of this process, the connection of the sewage, pumping station will begin. And the next step will be the restoration of wells at the drinking water intake," he said. 

    The Zaporizhzhia nuclear reactor complex was captured by Russia shortly after it launched its invasion of Ukraine six months ago.  

    Disconnecting the plant from Ukraine’s power grid would deny the country a major energy source. It is heavily reliant on nuclear power, with plants providing around 51 percent of the country’s electricity in 2020, according to the International Atomic Energy Agency.

    But a disruption of electricity supply to the plant could also knock out cooling systems that are essential for the safe operation of the reactors, while emergency diesel generators are sometimes unreliable. 

    The head of the International Atomic Energy Agency, the U.N.’s nuclear watchdog, said Thursday he hopes to send a mission to the plant within “days.”

    Negotiations over how the mission would access the plant are complicated but advancing, Rafael Mariano Grossi said on France-24 television after meeting in Paris with French President Emmanuel Macron, who pressed Russian President Vladimir Putin in a phone call last week to allow the U.N. agency to visit the site.

    “Kyiv accepts it. Moscow accepts it. So we need to go there,” Grossi said.