Russian military forces have seized control of the Chernobyl nuclear power plant, Ukrainian officials say.
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’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.”
The Ukrainian Ministry of Foreign Affairs has also warned of the possibility of “another ecological disaster” at the site.
Chernobyl’s “exclusion zone” – a 32 km (19 mile) 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 forces are part of Russian President Vladimir Putin’s “special military operation” in their neighbouring country.
Chernobyl is located about 130 km (80 miles) north of the capital Kyiv and could provide a path into the city for the invading forces.