Power has been knocked out to cooling systems at Chernobyl, the site of the world's biggest nuclear disaster, which is under Russian control.
The International Atomic Energy Agency says there should be no critical impact, although Ukrainian officials fear radiation leakage if electricity's not restored.
There appears to have been little progress evacuating people from besieged cities - with Ukrainian authorities claiming Russia has continued shelling Mariupol.
President Volodymyr Zelenskyy's warned there'll be a 'humanitarian catastrophe' unless it can keep skies above Ukraine safe.
He was responding after the US rejected an offer from Poland to donate fighter jets to its neighbour.
Volodymyr Zelenskyy has insisted there's no time for such back and forth.
"It's not a ping pong game. We're talking about people's lives, make this decision quickly. Please send us the fighter jets"
