President Volodymyr Zelensky of Ukraine called the damage “significant” but said there were no signs of radiation leaks. A Kremlin spokesman denied that Russia had carried out the strike.

Ukraine said a Russian drone crashed into the containment structure, starting a fire, but so far radiation levels have stayed constant.

A Russian drone struck the former nuclear power plant at Chernobyl in an attack overnight into Friday, Ukrainian President Volodymyr Zelensky has said. The strike hit the concrete shelter that covers the plant’s destroyed former fourth power unit,