A drone armed with a warhead hit the protective outer shell of Ukraine's Chernobyl nuclear plant early Friday, punching a hole in the structure and briefly starting a fire, in an attack Kyiv blamed on Russia.

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.