File photo: This handout photo taken from video and released by Russian Defense Ministry Press Service on Aug. 7, 2022, shows a general view of the Zaporizhzhia Nuclear Power Station in territory under Russian military control, southeastern Ukraine. AP
"Due to a high-tension line being cut, the plant lost its external electricity supply," the Russian administration wrote on Telegram, adding the causes of the outage were being investigated and that back-up diesel generators were keeping it working.
Ukraine's nuclear agency Energoatom accused Russia of carrying out an attack on Monday morning that caused the power cut, saying it was the seventh instance of the plant entering "blackout mode" since Moscow's troops took control in March 2022.
Energoatom said the generators had enough fuel reserves to last 10 days.
"If it is impossible to restore external power to the plant during this time, an accident with radiation consequences for the whole world may occur," it warned.
The governor of the Dnipro region said an overnight aerial attack had been reported, with four Russian missiles and 15 drones downed and at least eight civilians injured.
The last power cut at Zaporizhzhia, Europe's largest nuclear plant located in southeastern Ukraine, had been caused by another wave of Russian missile attacks, Energoatom said.
The external power supply was restored after a few hours on that occasion.
The UN's nuclear chief Rafael Grossi, who has tried to negotiate with both sides to reach a deal on the safety of the plant, said it was the seventh power cut at the huge nuclear facility during the war.
"Nuclear safety situation at the plant extremely vulnerable," he said on Twitter.
"We must agree to protect plant now; this situation cannot continue."
Grossi visited the Moscow-occupied plant in March.
Zaporizhzhia used to supply around 20 percent of Ukraine's electricity and continued to function in the early months of Russia's offensive despite frequent shelling, before halting power production in September.
None of its six Soviet-era reactors has since generated electricity, but the facility remains connected to the Ukrainian power grid for its own needs, notably to cool the reactors.