Russian military forces have captured the Chernobyl nuclear power plant.
A battle for control of the site - near Pripyat - followed fierce clashes with government forces, according to the Ukrainian Prime Minister Denys Shmyhal.
The current status of the power plant's 'confinement building', as well as a nuclear waste storage facility, is unknown.
It was the site of the world's worst nuclear disaster in 1986.
An adviser to the presidential office, Mykhailo Podolyak, said: "It is impossible to say the Chernobyl nuclear power plant is safe after a totally pointless attack by the Russians."
Ukraine's President Volodymyr Zelenskyy earlier said on Twitter that Russian forces were trying to seize the Chernobyl plant and his troops were "giving their lives so that the tragedy of 1986 will not be repeated".
He added: "This is a declaration of war against the whole of Europe."
Russian occupation forces are trying to seize the #Chornobyl_NPP. Our defenders are giving their lives so that the tragedy of 1986 will not be repeated. Reported this to @SwedishPM. This is a declaration of war against the whole of Europe.
— Володимир Зеленський (@ZelenskyyUa) February 24, 2022
Russia launched a “full-scale invasion” of Ukraine on Friday, with air strikes on major cities and troops arriving from the south and east.
Human rights worker Nadia Dobrianska is just outside the Ukrainian capital, Kyiv. She told The Hard Shoulder it has been a long day.
"After 5am when I was packing my things, I heard two explosions.
"And then when I was walking to my parents, who thankfully live down the street from me, I heard a third explosion."
She says she is close to a military airport, where she has heard "severe fighting around".
"I've heard the airplanes and Russian helicopters around - there's been a lot of fighting.
"The biggest issue is in the south, where Russia has taken over close to the Crimea.
"They've installed Russian flags - the east is more or less under control."
'Airstrikes on Kyiv'
Nadia says the area around Chernobyl is basically an open border.
"The worse thing that I've heard recently is that Russians have entered Chernobyl zone, and God forbid what may happen next.
"The Russian army has been training in there, anticipating their attack from Chernobyl.
"It's basically an open border out there, it's an alienation zone".
And she says they have been warned things will get worse.
"The president has called the general mobilisation now. So apparently the men, who I'm staying with, will have to go and serve.
"I don't want to sound those clichés like it's very uncertain times for me - my horizon of planning is literally two hours, maybe three.
"And I'm not sure I will be able to fall asleep tonight.
"We've been warned, as human rights organisations, that there's going to be airstrikes on Kyiv - massive ones - coming soon, anytime".
Additional reporting: IRN