Parts of Ukraine's Zaporizhzhia nuclear power plant were "seriously damaged" by military strikes that forced one of its reactors to shut down, the plant's operator said Saturday as the head of the United Nations' nuclear watchdog said he was "alarmed" by the shelling at Europe's largest such facility.

The Friday strikes on the Zaporizhzhia nuclear plant in south Ukraine – Europe's largest atomic power complex – "seriously damaged" a station containing nitrogen and oxygen and an "auxiliary building," Energoatom said on the Telegram messaging service.

Kyiv and Moscow blamed each other for the attacks.

The strikes had damaged a power cable, forced one of the reactors to stop working and "there are still risks of leaking hydrogen and radioactive substances, and the risk of fire is also high," said Energoatom.

The shelling had "caused a serious risk for the safe operation of the plant."

Russian troops have occupied the Zaporizhzhia plant since the early days of their invasion and Kyiv has accused them of storing heavy weapons there.

Moscow has accused Ukrainian forces of targeting the plant.

The European Union hit out at Russia on Saturday over the shelling.

"The EU condemns Russia's military activities around #Zaporizhzhia nuclear power plant," the bloc's top diplomat, Josep Borrell, wrote on Twitter.

"This is a serious and irresponsible breach of nuclear safety rules and another example of Russia's disregard for international norms."

Borrell insisted that the U.N.'s nuclear watchdog, the International Atomic Energy Agency (IAEA), be given access to the plant.

The U.N.'s nuclear watchdog also expressed its alarm in a statement Saturday.

The strikes represented "the latest in a long line of increasingly alarming reports," said Rafael Grossi, director general of the IAEA.

They underlined "the very real risk of a nuclear disaster that could threaten public health and the environment in Ukraine and beyond."

He added that "military action jeopardizing the safety and security" of the plant was "completely unacceptable and must be avoided at all costs."

The IAEA has been trying to send a team to inspect the plant for weeks. Ukraine has so far rejected the efforts, which it says would legitimize Russia's occupation of the site in the eyes of the international community.

It said employees of Russian nuclear operator Rosatom had left the plant shortly before the attacks but that Ukrainian personnel had stayed on and the plant was still generating electricity.

Ukraine President Volodymyr Zelenskyy had said on Friday that "any bombing of this site is a shameless crime, an act of terror."

And the Ukrainian Foreign Ministry had said that the "possible consequences of hitting a working reactor are equivalent to using an atomic bomb."

Earlier in the week, the IAEA described the situation at the nuclear power plant as "volatile."

"Every principle of safety has been violated one way or the other," said Grossi.