Local authorities reported no immediate radiation rise was detected and "essential" equipment was unaffected by the fire, but it remained unclear what the invading forces planned next.

President Volodymr Zelensky accused Moscow of trying to "repeat" the Chernobyl nuclear disaster and said he had spoken with international leaders, including US President Joe Biden, about the crisis at the Zaporizhzhia nuclear plant.

Biden urged Russia to allow emergency responders to go to the site.

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Images on a live feed from the site earlier showed blasts lighting up the night sky and sending up plumes of smoke, with the International Atomic Energy Agency urging an immediate halt to fighting there.

"No country other than Russia has ever fired on nuclear power units," Zelensky said in a video message.

"This is the first time in our history. In the history of mankind. The terrorist state now resorted to nuclear terror."

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Zelensky appealed for global help.

"If there is an explosion, it is the end of everything. The end of Europe. This is the evacuation of Europe. Only immediate European action can stop Russian troops," he said. 

But after several hours of uncertainty, Ukrainian authorities said the site had been secured.

AFP/eNCA