KYIV, Ukraine (AP) — The wall of a major dam in a part of southern Ukraine that Moscow controls collapsed Tuesday, triggering floods, endangering Europe’s largest nuclear power plant and threatening drinking water supplies as both sides in the war rushed to evacuate residents and blamed each other for the emergency.

Ukraine accused Russian forces of blowing up the Kakhovka dam and hydroelectric power station on the Dnieper River, while Russian officials blamed Ukrainian military strikes in the contested area. It was not possible to verify the claims.

The potentially far-reaching environmental and social consequences of the disaster quickly became clear as homes, streets and businesses flooded downstream and emergency crews began evacuations; officials raced to check cooling systems at the Zaporizhzhia Nuclear Power Plant; and authorities expressed concern about supplies of drinking water to the south in Crimea, which Russia illegally annexed in 2014.

The dam break added a stunning new dimension 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 1,000 kilometers (621 miles) of frontline in the east and south of Ukraine.

It was not immediately clear whether either side benefits from the damage to the dam, since both Russian-controlled and Ukrainian-held lands are at risk of flooding. The damage could also hinder Ukraine’s counteroffensive in the south and distract its government, while at the same time Russia depends on the dam to supply water to Crimea.

Patricia Lewis, director of the International Security Program at Chatham House think tank in London, said apportioning blame is difficult but “there are all sorts of reasons why Russia would do this.”

“There were reports (last fall) of Russians having mined the reservoir. The question we should pose is why the Ukrainians would do this to themselves, given this is Ukrainian territory,” she said.

Experts have previously said the dam structure was suffering from disrepair. David Helms, a retired American scientist who has monitored the reservoir since the start of the war, wrote in an e-mail that it wasn’t clear if the damage was deliberate or simple neglect from Russian forces occupying the facility.

But Helms reserved judgement, noting as well a Russian history of attacking dams.

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 at 2.50 a.m. (2350 GMT) and said some 80 settlements were in danger.

But Kremlin spokesman Dmitry Peskov called it “a deliberate act of sabotage by the Ukrainian side … aimed at cutting water supplies to Crimea.”