Zaporizhzhia power plant supplies an estimated 40 percent of Ukraine's nuclear power.
A fire was reported in a key nuclear plant in Ukraine on Friday as Russia intensified fighting deep inside its western neighbour. "No country other than Russia has ever fired on nuclear power units," Ukraine's President Volodymyr Zelensky said as he accused Moscow of resorting to "nuclear terror" and wanting to "repeat" the Chernobyl disaster.
The attack on Zaporizhzhya nuclear power plant, the biggest in Europe, raised concerns that radiation could leak from the damaged power station.
"No country other than Russia has ever fired on nuclear power units. This is the first time in our history. In the history of mankind. The terrorist state now resorted to nuclear terror," Zelensky said in a video message.
US President Joe Biden joined Zelenskiy "in urging Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site," the White House said in a statement.
Ukrainian foreign minister Dmytro Kuleba, meanwhile, called on Russian troops to stop the attack.
"If it blows up, it will be 10 times larger than Chornobyl! Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone," Mr Kuleba tweeted.
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.
After a string of setbacks, the Russian forces have taken the Black Sea port of Kherson in southern Ukraine. They have also encircled the strategic port city of Mariupol, which is without water or electricity.
Since Russia launched the military offensive last Thursday, over a million refugees have fled Ukraine.
Russian President Vladimir Putin has said that the invasion of Ukraine is "going to plan" as Russia was rooting out "neo-Nazis", and that he will never "give up on (his) conviction that Russians and Ukrainians are one people".