Ukrainian President Volodymyr Zelensky has expressed serious concern about the overnight fire at Zaporizhia nuclear power plant.

"This threatens to be the second Chornobyl and an even larger one," Zelensky said in a video address.

"We don't know how the fire at the plant will end, when there will be an explosion or when it won't happen, God willing. No one will calculate that to the end," the president said.

"We have contacted the leaders, our partners. I spoke with [European Council President] Charles Michel, with [German Chancellor Olaf] Scholz, I spoke with [Polish President Andrzej] Duda, I spoke with [United States] President [Joe] Biden. We have contacted of IAEA [International Atomic Energy Agency] head [Rafael] Grossi, as well as [British] Prime Minister [Boris] Johnson. We warn everybody," Zelensky said.

"Not a single state, except Russia, has ever fired at nuclear power units. For the first time in our history, in the history of mankind, the terrorist state has also resorted to nuclear terror," Zelensky said in a video message on Friday night.

"Russian propagandists threatened to cover the world with nuclear ashes. Now this is not a threat, but a reality, and we do not know how the fire at the station will end, when there will be an explosion or when it will not happen, God forbid. No one will ever figure this out. But our guys always kept the nuclear plant safe so that there were no provocations, so that no one could enter the plant, so that no one could seize this plant, mine it and then blackmail the whole world with a nuclear catastrophe," the president said.

"We must stop the Russian military immediately! Shout out to your politicians! Ukraine has 15 nuclear power units, if there is an explosion, it will be the end of everything, the end of Europe. This is the evacuation of Europe. Only immediate European action can stop Russian forces. Don't let Europe die from a disaster at a nuclear power plant," Zelensky called on Europeans and people around the world.