Russia’s military says it has taken control of the Zaporizhzhia power plant in Ukraine after its forces opened fire on the facility overnight with rockets, artillery and gunfire. The assault sparked a fire that burned for hours, though it reportedly did not spread to any of the plant’s six reactors, and there’s no sign of an increase in radiation levels at the site. The nuclear plant’s operator says managers are now “working at gunpoint.”

Zaporizhzhia is the largest nuclear power plant in Europe and among the 10 largest in the world. Nuclear experts have been warning for weeks of the risk of a catastrophic meltdown. News of the assault drew international alarm. British Prime Minister Boris Johnson called for an emergency meeting of the U.N. Security Council. U.S. Senator Ed Markey of Massachusetts tweeted, “This would be an international war crime by Putin that could result in incredible devastation.” Ukraine’s President Volodymyr Zelensky accused Russia of “nuclear terror.”

President Volodymyr Zelensky: “Europeans, please wake up. Tell your politicians: Russian troops are shooting at a nuclear power plant in Ukraine, Zaporizhzhia nuclear power plant, city of Enerhodar. There are six nuclear reactors there. Six. In Chernobyl, it was only one reactor which exploded.”