Ukraine’s nuclear ghosts have been stirred up again after a drone armed with a warhead hit the protective outer shell of the plant at Chernobyl, the site of the world’s worst nuclear accident. The strike on Friday did not breach the plant’s inner containment shell and radiation levels did not increase.

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.