Ukraine’s Zaporizhzhia Nuclear Power Plant has lost all of its external power for the second time in five days, the head of the International Atomic Energy Agency (IAEA) said Wednesday, decrying the “worrying” news at Europe’s largest nuclear plant.

“This repeated loss of #ZNPP’s off-site power is a deeply worrying development and it underlines the urgent need for a nuclear safety & security protection zone around the site,” Rafael Grossi said on Twitter.

Its backup diesel generators are now providing electricity for its nuclear safety and security functions, he added – both vital for preventing possible accidents at the plant, which is now surrounded by Russia troops.

According to Ukraine's National Atomic Energy Generating Company Energoatom, the plant’s Dniprovska substation in the Dnipropetrovsk region was damaged as a result of a rocket fire by Russian troops.

“As a result, the Zaporizhzhia NPP was completely de-energized. Diesel generators turned on automatically,” Energoatom said on Telegram.

Noting that Energoatom sent another batch of diesel fuel to the plant, it said that as of 10.00 a.m. local time (0700GMT) the Russian side has not let the company's vehicle convoy pass.

Russian shelling and damage to energy infrastructure associated with the operation of nuclear power plants exhibits the same “nuclear terrorism” as direct shelling of the Zaporizhzhia plant, and leads to the same consequences and threats of a radiation accident, it added.

The occupiers continue to neglect the nuclear and radiation safety of the largest nuclear power plant in Europe, threatening the world with a radiation disaster, it said.

Under Russian control since March, the plant in southeastern Ukraine was disconnected from the country’s power grid last Monday, amid growing concerns of a nuclear disaster as Moscow and Kyiv accuse each other of attacks on the nuclear facility.

Energoatom said a power line was restored on Saturday to allow officials to “shut down power unit No. 6 and transfer it to the safest state – cold shutdown.”

The agency warned that the risk of more damage and outside power being cut again “remains high.”

In such a scenario, the plant will have to be “powered by diesel generators, the duration of which is limited by the technological resource and the amount of available diesel fuel,” the statement added.

Zaporizhzhia, one of the world’s 10 largest nuclear power plants, generated 20% of Ukraine’s electricity before the war.