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Ukraine's nuclear operator, Energoatom, on Tuesday reported an "unprecedented" cyberattack on its website, but said its operations had not been disrupted, AFP reported.

"On August 16, 2022, the most powerful cyberattack since the start of the Russian invasion occurred against Energoatom's website," the operator said on Telegram, adding the website "was attacked from Russian territory".

The Russian "popular cyberarmy" group used more than 7 million internet bots to attack the website for three hours, Energoatom said.

However, it added, the assault "did not have a considerable impact on the work of the Energoatom website".

A Telegram channel called "popular cyberarmy" in Russian around midday called on its followers to attack the Ukrainian nuclear operator's website, according to AFP.

But by Tuesday evening, it had announced a "change" in plans, redirecting supporters to a new target -- the Ukrainian Institute of National Remembrance, whose website was sluggish.

The cyberattack comes as tensions flare over the Zaporizhzhia power plant in southern Ukraine, which Russian forces occupied in March, shortly after the start of the Russian war on Ukraine.

Russia and Ukraine have accused each other of shelling the nuclear installation, which is the largest in Europe, sparking fears of a nuclear accident.

Last Friday, the UN demanded an end to the shelling of the Zaporizhzhia nuclear power plant. Previously, International Atomic Energy Agency director general Rafael Grossi warned that the Zaporizhzhia nuclear plant is “completely out of control.”

At the start of the Ukraine war, Russian forces had also seized the spent nuclear fuel and radioactive waste facilities at Chernobyl, the site of the world's worst nuclear accident at a now-defunct power plant.