Yuri Chernichuk, the facility's chief engineer, said on Wednesday that he had agreed to take the post because it was "in my opinion the only correct decision."

Rosenergoatom, the Russian nuclear operator put in charge by Moscow, confirmed his appointment.

On Thursday, Ukraine's nuclear agency Energoatom formally sacked Chernichuk for "collaborative and treasonous activities".

Petro Kotin, who heads Energoatom, said Chernichuk had "betrayed Ukraine" and gone "over to the side of the enemy".

"Instead of making every effort to free the station as soon as possible, he decided to help Russian occupiers legalise its criminal seizure and now incites other workers to do the same," Kotin said on Telegram.

"The Russian occupiers are putting more and more pressure on the pro-Ukrainian staff of the plant, intimidating and blackmailing you. But there is only one decent way out -- to hold on!" he said.

"The transition to Russian structures will make you direct assistants of the aggressor, and therefore enemies of your own compatriots."

Russia seized the Zaporizhzhia power plant in southern Ukraine soon after invading the country in February.

Russia and Ukraine have for months accused each other of shelling around the plant.

The UN's atomic watchdog in November denounced strikes on the plant's territory, calling for a "stop to this madness."