Ukraine's Zaporizhzhia nuclear power plant, the biggest in Europe, has lost its last remaining external power source as a result of renewed shelling and is now relying on emergency diesel generators, the U.N. nuclear watchdog said Saturday.

The International Atomic Energy Agency said the plant's link to a 750-kilovolt line was cut at around 1 a.m. Saturday. It cited official information from Ukraine as well as reports from IAEA experts at the site, which is held by Russian forces.

All six reactors at the plant are shut down but they still require electricity for cooling and other safety functions. Plant engineers have begun work to repair the damaged power line and the plant's generators — not all of which are currently being used — each have sufficient fuel for at least 10 days, the IAEA said.

"The resumption of shelling, hitting the plant's sole source of external power, is tremendously irresponsible," IAEA Director-General Rafael Grossi said in a statement.

Grossi visited Kyiv on Thursday. He said he will soon travel to Russia, then make another trip to Ukraine, to further his effort to set up a "nuclear safety and security protection zone" around the plant, which he has advocated for weeks.

"This is an absolute and urgent imperative," he said. The IAEA didn't apportion blame for the shelling.

Zaporizhzhia is one of four regions that Russian President Vladimir Putin has annexed in violation of international laws. While the nuclear plant has been under Russian control for months, the city of the same name remains under Ukrainian control.

Putin signed a decree Wednesday declaring that Russia was taking over the plant. Ukraine's Foreign Ministry called it a criminal act and said it considered Putin's decree "null and void." Ukraine's state nuclear operator, Energoatom, said it would continue to operate the plant.

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