Russia has seized control of the Chernobyl nuclear exclusion zone after an intense fight over the area, the Ukrainian government has said.
Mykhailo Podolyak, an adviser to the Ukrainian presidential office, said: "It is impossible to say the Chernobyl nuclear power plant is safe after a totally pointless attack by the Russians."
"This is one of the most serious threats in Europe today."
Podolyak gave the update at around 5.30pm UK time, after Ukrainian President Volodymyr Zelenskyy said the area was under attack at 3pm.
Zelenskyy said: "Our defenders are giving their lives so that the tragedy of 1986 will not be repeated."
Earlier, an advisor to Ukraine's interior minister warned if any strikes from the invaders damage the nuclear waste collectors, then "nuclear dust can be spread over the territory of Ukraine, Belarus and the country of the EU".
The Ukrainian Foreign Ministry echoed Zelenskyy's warning, tweeting that a Russian attack on Ukraine could “cause another ecological disaster.”
The exclusion zone sits just across the border from Belarus, and it is understood that troops entered the area through the country, which is closely tied to Russia.
The Chernobyl plant is the site of the world's worst nuclear disaster, which took place in 1986 after one of its reactors exploded.
The plant sits on the border with Belarus and is directly north of Kyiv.
The development comes as the Ukraine faced a bombardment by land, air and sea after Russia launched a full-scale invasion in the early hours of Thursday morning.
The attack is the biggest experienced by a European country since World War II.
Troops have poured across the borders with Russia and Belarus from the north and east, and landed on the coasts from the Black Sea in the southwest and Azov Sea in the southeast.
The UK's ministry of defence confirmed that heavy casualties have been seen on both sides, but cautioned that exact numbers are not clear.
Zelenskyy said Ukraine was listening to the sound of a new iron curtain falling as Russian troops advanced across his country.
The invasion has been widely condemned by the wider world, with the UK, US, EU and Nato all promising severe sanctions in response to its aggression.
Watch: Russia launches invasion of Ukraine - forces 'trying to seize' site of Chernobyl nuclear disaster
Boris Johnson unveiled new sanctions against Russia on Thursday in a bid to cripple the economy.
The PM extended punitive measures on Thursday to hit five further oligarchs, including the Russian President’s former son-in-law, and to tackle more than 100 businesses and individuals.
He told the House of Commons: “Putin will stand condemned in the eyes of the world and of history. He will never be able to cleanse the blood of Ukraine from his hands.
“Now we see him for what he is – a bloodstained aggressor who believes in imperial conquest.”
Johnson said the measures are “the largest and most severe package of economic sanctions that Russia has ever seen”, but vowed to go further.
“We will continue on a remorseless mission to squeeze Russia from the global economy piece by piece, day by day and week by week,” he told MPs.