The wall of a major dam in southern Ukraine 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 destruction.

Ukraine accused Russian forces of blowing up the Kakhovka dam and hydroelectric power station on the Dnieper River in an area that Moscow controls, while Russian officials blamed Ukrainian bombardment 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.

Both Russian and Ukrainian authorities brought in trains and buses for residents. About 22,000 people live in areas at risk of flooding in Russian-controlled areas, while 16,000 live in the most critical zone in Ukrainian-held territory, according to official tallies. Neither side reported any deaths or injuries.

The dam break added a stunning new dimension to Russia’s 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 front line in the east and south.

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. The damage could also hinder Ukraine’s counteroffensive in the south and distract its government, while 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 was suffering from disrepair. David Helms, a retired American scientist who has monitored the reservoir since the start of the war, said in an e-mail that it wasn’t clear if the damage was deliberate or simple neglect from Russian forces occupying the facility.

Russia’s History of Attacking Dams Noted

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

Authorities, experts and residents have expressed concern for months about water flows through — and over — the Kakhovka dam. After heavy rains and snow melt last month, water levels rose beyond normal levels, flooding nearby villages. Satellite images showed water washing over damaged sluice gates.

Amid official outrage, Ukrainian President Volodymyr Zelenskyy said he convened an urgent meeting of the National Security Council. He alleged Russian forces set off a blast inside the dam structure at 2:50 a.m. and said about 80 settlements were in danger. Zelenskyy said in October his government had information that Russia had mined the dam and power plant.

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

Both sides warned of a looming environmental disaster. Ukraine’s Presidential Office said some 150 metric tons of oil escaped from the dam machinery and that another 300 metric tons could still leak out.

Andriy Yermak, the head of Ukraine’s President’s Office, posted a video showing swans swimming near an administrative building in the flooded streets of Russian-occupied Nova Kakhovka, a city in the Kherson region where some 45,000 people lived before the war. Other footage he posted showed flood waters reaching the second floor of the building.

Ukraine’s Interior Ministry urged residents of 10 villages on the Dnieper’s right bank and parts of the city of Kherson downriver to gather essential documents and pets, turn off appliances, and leave, while cautioning against possible disinformation.

The Russian-installed mayor of occupied Nova Kakhovka, Vladimir Leontyev, said it was being evacuated as water poured into the city.

Ukraine’s nuclear operator Energoatom said in a Telegram statement that the damage to the dam “could have negative consequences” for the Zaporizhzhia Nuclear Power Plant, which is Europe’s biggest, but wrote that for now the situation is “controllable.”

The U.N.’s International Atomic Energy Agency said in a statement there was “no immediate risk to the safety of the plant,” which requires water for its cooling system.

It said that IAEA staff on site have been told the dam level is falling by 2 inches an hour. At that rate, the supply from the reservoir should last a few days, it said.

The plant also has alternative sources of water, including a large cooling pond than can provide water “for some months,” the statement said.

Ukrainian authorities have previously warned that the dam’s failure could unleash 18 million cubic meters (4.8 billion gallons) of water and flood Kherson and dozens of other areas where thousands of people live.

The World Data Center for Geoinformatics and Sustainable Development, a Ukrainian nongovernmental organization, estimated that nearly 100 villages and towns would be flooded. It also reckoned that the water level would start dropping only after five-seven days.

A total collapse in the dam would wash away much of the broad river’s left bank, according to the Ukraine War Environmental Consequences Working Group, an organization of environmental activists and experts documenting the war’s environmental effects.

Mykhailo Podolyak, a senior adviser to President Volodymyr Zelenskyy, said that “a global ecological disaster is playing out now, online, and thousands of animals and ecosystems will be destroyed in the next few hours.”

Video posted online showed floodwaters inundating a long roadway; another showed a beaver scurrying for high ground from rising waters.

The incident also drew international condemnation, including from German Chancellor Olaf Scholz and NATO Secretary-General Jens Stoltenberg, who said the “outrageous act … demonstrates once again the brutality of Russia’s war in Ukraine.”

Ukraine controls five of the six dams along the Dnieper, which runs from its northern border with Belarus down to the Black Sea and is crucial for the country’s drinking water and power supply.

Ukraine’s state hydro power generating company wrote in a statement that “The station cannot be restored.” Ukrhydroenergo also claimed Russia blew up the station from inside the engine room.

Leontyev, the Russian-appointed mayor, said numerous Ukrainian strikes on the Kakhovka hydroelectric plant destroyed its valves, and “water from the Kakhovka reservoir began to uncontrollably flow downstream.” Leontyev added that damage to the station was beyond repair, and it would have to be rebuilt.

Ukraine and Russia have previously accused each other of targeting the dam with attacks.