The wall of a major dam in a part of southern Ukraine that Moscow controls collapsed Tuesday after a reported explosion, sending water gushing downriver and prompting dire warnings of ecological disaster as officials from both sides in the war ordered residents to evacuate.

Ukraine accused Russian forces of blowing up the dam and hydroelectric power station, while Russian officials blamed Ukrainian military strikes in the contested area.

The fallout could have broad consequences: Flooding homes, streets and businesses downstream; depleting water levels upstream that help cool Europe’s largest nuclear power plant; and draining supplies of drinking water to the south in Crimea, which Russia illegally annexed.

The dam break added a complex new element to Russia’s ongoing war in Ukraine, now in its 16th month. Ukrainian forces were widely seen to be moving forward with a long-anticipated counteroffensive in patches along more than 1000 kilometers (621 miles) of frontline in the east and south of Ukraine.

Amid official outrage, Ukrainian President Volodymyr Zelenskyy said he convened an urgent meeting of the National Security Council. He alleged that Russian forces set off a blast inside the dam structure and said some 80 settlements were in danger.