The overnight strike at the nuclear plant, which is the site of the world's worst nuclear accident, caused a fire that has since been extinguished, he added. As of Friday morning, radiation levels inside and outside Chernobyl remain normal and stable according to the UN's nuclear watchdog (the IAEA). Russia has denied any claims it attacked Chernobyl, stating its military does not strike Ukrainian nuclear infrastructure and "any claims that this was the case do not correspond to reality". The International Atomic Energy Agency (IAEA), which monitors nuclear safety around the world, said fire safety personnel and vehicles responded within minutes to an overnight explosion. No casualties were reported, the agency added. The agency remains on "high alert" after the incident, with its director general Rafael Mariano Grossi saying there is "no room for complacency".