Russian forces are firing rockets at a physics institute in the city of Kharkiv that contains nuclear material and a reactor, Ukraine’s national security service has said.
A strike on the nuclear facility could lead to “large-scale ecological disaster,” the security service said.
Writing on Facebook on Sunday the service said that Russians were firing from Grad launchers.
Those missiles do not have precise targeting, raising concern that one would go astray.
Ukrainian President Voldymyr Zelenskyy reiterated a request for foreign protectors to impose a no-fly zone over Ukraine.
Nato has so far has ruled this due to concerns the West could be dragged into a war with Russia.
“The world is strong enough to close our skies,” Zelenskyy said in a video address on Sunday.
Russian President Vladimir Putin warned Saturday that Moscow would consider a third-party declaration to close Ukrainian airspace to be a hostile act.
It comes after Russian forces shelled the Zaporizhzhia nuclear plant in Enerhodar on Thursday, sparking fears of a “nuclear catastrophe”.
Security footage from the site showed a blaze breaking out in one of the buildings, prompting alarm across the world before the fire was extinguished and officials said the facility was safe.
The US and its allies heavily criticised Russia on Friday at the UN over its shelling of the plant, with 15 envoys expressing “grave concern” and shock.
Linda Thomas-Greenfield, the US ambassador to the United Nations, warned on Saturday that Putin’s forces were closing in on a plant in Yuzhnoukrainsk.
She said that nuclear facilities “cannot become part of this conflict”, adding: “Reliable electricity is vital for the nuclear facility, as are back-up diesel generators and fuel. Safe transit corridors must be maintained.
“Russia must halt any further use of force that might put at further risk all 15 operable reactors across Ukraine – or interfere with Ukraine’s ability to maintain the safety and security of its 37 nuclear facilities and their surrounding populations.”