Ukraine’s closed Chernobyl nuclear power plant has been disconnected from the nation’s power grid by Russian forces, Ukraine’s state-owned grid operator Ukrenergo said Wednesday, potentially jeopardizing the cooling of nuclear material stored at the site.

“Because of military actions of Russian occupiers the nuclear power plant in Chornobyl was fully disconnected from the power grid. Nuclear station has no power supply,” Ukrenergo said in a statement on its official Telegram page, using Ukraine’s spelling for the plant.

Electricity is needed for cooling, ventilation and fire-extinguishing systems at the closed site. In a statement on its Facebook page, Ukrenergo also said that emergency diesel generators have been turned on but that the fuel would last for only 48 hours.

Ukrainian Foreign Minister Dmytro Kuleba on Wednesday demanded a cease-fire with Russia to allow repairs.

“The only electrical grid supplying the Chornobyl NPP and all its nuclear facilities occupied by Russian army is damaged,” he tweeted. “… I call on the international community to urgently demand Russia to cease fire and allow repair units to restore power supply.”

He warned that after reserve diesel generators run out of fuel, “cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent. Putin’s barbaric war puts entire Europe in danger.”

The International Atomic Energy Agency said Wednesday on Twitter that the power loss “violates [a] key safety pillar on ensuring uninterrupted power supply,” but it added that “in this case IAEA sees no critical impact on safety.” The agency said factors including the volume of cooling water at Chernobyl were “sufficient for effective heat removal without the need for electrical supply.”

The IAEA also said Tuesday that it had lost contact with monitoring systems that transmit data on nuclear material at the Chernobyl plant.

The U.N. nuclear watchdog’s director-general “indicated that remote data transmission from safeguards monitoring systems installed at the Chornobyl NPP had been lost,” the IAEA said in a statement.

“The Agency is looking into the status of safeguards monitoring systems in other locations in Ukraine and will provide further information soon,” it added.

Ukrainian presidential adviser Mykhailo Podolyak tweeted Wednesday that the IAEA had “unexpectedly lost connection” with the monitoring systems, calling it an “extremely dangerous situation.”

The Chernobyl plant, then under the Soviet Union’s control, was the scene of a 1986 disaster in which explosions and fires sent a huge radioactive cloud over parts of Europe and left contaminated soil and other fallout, which remains dangerous, at the plant site.

The catastrophe ranks as the world’s worst nuclear power plant accident.

Last month, the Russian Defense Ministry confirmed that its forces had taken control of the area near the site as part of Russia’s wider invasion of Ukraine, sparking global alarm.

The European Union said in a statement Wednesday that it was “extremely concerned” by the nuclear safety risks “caused by the Russian invasion on Ukraine and the potential damage to its nuclear facilities.”

It issued an “urgent call” for Russia to end its military operations near all nuclear power facilities in Ukraine and to allow their safe operation.

“We must do everything that we can to prevent a nuclear accident, incident, or another radiological emergency that could seriously impact local populations, neighboring countries, and the international community. It is time to act to avoid such a scenario,” the statement said.