The BBC, citing Ukrainian officials, says Russian military forces have seized control of the Chernobyl nuclear power plant, Ukrainian officials say.
Presidential adviser Mykhailo Podoliak reportedly 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's president 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," President Volodymyr Zelensky wrote earlier on Twitter.
"This is a declaration of war against the whole of Europe."
Chernobyl's "exclusion zone" – a 32 km 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 that helped make the site a tourist attraction.
Russian troops reportedly entered the exclusion zone earlier on Thursday before crossing over into Ukraine.
The White House says it has received reports that staff are being held hostage at the site by Russian soldiers.
Meanwhile, TASS notes that the International Atomic Energy Agency (IAEA) Director General Rafael Grossi said in a statement on Thursday that Ukraine notified the IAEA that "unidentified armed forces" seized control over the Chernobyl nuclear power plant, the station itself sustained no damage.
