VIENNA (Reuters) -All external power lines supplying electricity to the Russian-occupied Zaporizhzhia Nuclear Power Plant in Ukraine were down for several hours on Friday, the U.N. nuclear watchdog said, but the station's management later said power had been restored.

The head of the International Atomic Energy Agency, Rafael Grossi, acknowledged that power had been restored after 3 1/2 hours. But he added in a statement on X that nuclear safety "remains extremely precarious in Ukraine."

Ukraine's energy minister blamed Russian shelling for severing the last power line to the plant and its six reactors. The country's power distribution operator said its technicians had taken action to restore it.

Europe's biggest nuclear power plant, which is not operating but still requires power to keep its nuclear fuel cool, switched during the outage to running on diesel generators, the IAEA said.

The organization has repeatedly warned of the risk of a catastrophic accident at Zaporizhzhia, which is located near the front line in the war in Ukraine. Its reactors are shut down, but the nuclear fuel inside them still needs to be cooled, which requires constant power.

The plant's Russia-installed management issued a statement on Telegram saying the high-voltage line to the plant had been restored.

The statement said there had been no disruptions to operations at the plant, no violations of security procedures and no rise in background radiation levels beyond normal levels.

The IAEA had earlier said that the plant had lost all off-site power for the ninth time during the military conflict and for the first time since late 2023. "The ZNPP currently relies on power from its emergency diesel generators, underlining (the) extremely precarious nuclear safety situation," it said.

Ukrainian Energy Minister German Galuschenko wrote on Telegram that a Russian strike had cut off the plant. 

"The enemy struck the power line connecting the temporarily occupied (Zaporizhzhia Nuclear Plant) with the integrated power system of Ukraine."

Ukrenergo, the sole operator of high-voltage lines in Ukraine, said its specialists had brought it back into service.

"Ukrenergo specialists have brought back into service the high-voltage line which supplies the temporarily occupied power station," it said on Telegram.

Neither the IAEA nor the plant's Russian-installed management initially cited a cause for the cut-off.

Russian forces seized the Zaporizhzhia station in the first weeks of Russia's February 2022 invasion of Ukraine. Each side regularly accuses the other of firing or taking other actions that could trigger a nuclear accident.

(Reporting by Francois Murphy in Vienna and Ronald Popeski in WinnipegEditing by Peter Graff and Matthew Lewis)