Russian military forces have seized control of the Chernobyl nuclear power plant, Ukrainian officials say.
A radiation sign is seen in Chernobyl, Ukraine. (File photo) Photo: Vladimir Sindeyeve/NurPhoto
Presidential adviser Mykhailo Podoliak said the "totally pointless attack" on Thursday amounted to "one of the most serious threats in Europe today".
An explosion at Chernobyl in 1986 led to the worst nuclear disaster in human history, both in cost and casualty.
Ukraine President Volodymyr Zelensky warned such a disaster could happen again if Russia continued its invasion.
"Our defenders are giving their lives so that the tragedy of 1986 will not be repeated," he wrote earlier on Twitter.
"This is a declaration of war against the whole of Europe."
The Ukrainian Ministry of Foreign Affairs also warned of the possibility of "another ecological disaster" at the site.
Chernobyl's "exclusion zone" - a 32km radius around the plant - remains largely devoid of life 36 years after a faulty reactor caused a major explosion at the plant.
The plant's three other reactors were all shut down by 2000 and it has since been decommissioned.
Radiation levels in the area remain dangerously high since the 1986 leak, chronicled in an eponymous HBO mini-series in 2019.
Russian troops reportedly entered the exclusion zone on Thursday before crossing over into Ukraine.
The White House said it had received reports that staff were being held hostage at the site by Russian soldiers.
The forces are part of Russian President Vladimir Putin's "special military operation" in their neighbouring country.
Chernobyl is located about 130km north of the Ukraine capital, Kyiv, and could provide a path into the city for the invading forces.
Samantha Turner, a security fellow at the Truman National Security Project, said control of the area did not have "battle-determining significance" but would give Russian forces a corridor to the Dnipro River.
The river runs north into Belarus, whose president has closely aligned himself with Putin, and south to Kyiv.
"It's an important part of them opening up different corridors for troop movement and controlling key terrain," Turner said.
She warned that while nobody lives in the area and the plant was no longer active, any active fighting over the territory could cause radioactive waste spillage.
However, University of Sheffield radioactive waste materials professor Claire Corkhill noted Russians were among the world's most experienced nuclear operators.
She has worked as part of the international clean-up effort at Chernobyl for the past six years, even visiting the site three times.
The most significant success in these collaborations was the recent construction of a 32,000-tonne dome around the radioactive reactor, funded at a cost of $US1.5b by more than 30 countries.
Corkhill worried the invasion of Ukraine would effectively pause the operations.
"Thirty years have passed since the accident and we've still not cleaned everything up," she told the BBC. "It's easily another 50-year programme."
"If people aren't properly working on that facility, and progressing the decommissioning, it could be a really big problem."
The 1986 explosion is often linked to the fall of the Soviet Union five years later.
Dr Taras Kuzio, research fellow at the Henry Jackson Society, said the seizure of Chernobyl was therefore best looked at as a symbolic win for Putin.
"Putin has the mindset of somebody who cannot get over the fact the USSR disintegrated 30 years ago, and it all began to disintegrate after Chernobyl," Dr Kuzio, who is of Ukrainian descent, said.
He feared that while Putin may simply be using the threat of nuclear weaponry to ward off the West, he was operating like a "sociopath" and his actions should be watched closely.
"What he's doing in Ukraine is unprecedented. Why should we assume he is not going to do anything else?" he said.
- BBC