The blaze erupted at a power unit of the Zaporizhzhia nuclear power plant following Russian strikes, according to spokesman Andrei Tuz.

The station at Zaporizhzhia, an industrial city in southeastern Ukraine, supplies an estimated 40 percent of the country's nuclear power.

Ukraine is fighting for its existence one week after Russian President Vladimir Putin announced an invasion to support Moscow-backed separatists in the east.

The Russian military has bombarded cities in Ukraine with shells and missiles, forcing civilians to cower in basements, including in Chernobyl, the site of the world's worst nuclear disaster.

But Kuleba warned that the fallout from a disaster at the Zaporizhzhia plant would be magnitudes worse.

"If it blows up, it will be 10 times larger than Chornobyl! Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone," Kuleba tweeted.

He wrote that the Russian forces were firing at the facility "from all sides".

The UN's International Atomic Energy Agency has already urged Russia to "cease all actions" at Ukraine's nuclear facilities, including the site of the 1986 Chernobyl disaster.