Russian shelling has cut power to the site of the Chernobyl nuclear plant, part of which was destroyed in a 1986 nuclear meltdown, Ukraine's energy ministry said.
Ukraine said the confinement structure housing a damaged reactor lost power in the incident, and that specialists were currently working to restore electricity.
Ukrainian President Volodymyr Zelensky accused Russia of posing a threat to global security.
"Every day that Russia prolongs the war, refuses to implement a full and reliable ceasefire, and continues striking all objects of our energy infrastructure - including those critical to the safety of nuclear power plants and other nuclear facilities - is a global threat," Mr Zelensky said on Facebook.
Ukraine did not elaborate on the consequences of the blackout, and Russia did not immediately comment.
"Due to Russian shelling of energy infrastructure in the Kyiv region, an emergency situation arose at the facilities of Chernobyl Nuclear Power Plant," Ukraine's energy ministry said in a statement on Telegram.
"As a result of power surges, the New Safe Confinement, a key facility that isolates the destroyed fourth power unit of Chernobyl Nuclear Power Plant and prevents the release of radioactive materials into the environment, was left without power," it added.
The New Safe Confinement, completed in 2016, is a large shield-like structure that surrounds the unit four reactor and prevents the release of radioactive material.
A Russian drone attack damaged the confinement structure in February, but did not result in increased radiation in the surrounding area, Ukrainian authorities said.
