A major dam in southern Ukraine collapsed Tuesday, flooding villages, endangering crops and threatening drinking water supplies as both sides in the war scrambled to evacuate residents and blamed each other for the destruction.

Ukraine accused Russian forces of blowing up the Kakhovka dam and hydroelectric power station, which sits on the Dnieper River in an area that Moscow has controlled for more than a year. Russian officials blamed Ukrainian bombardment in the contested area.

It was not possible to reconcile the conflicting claims.

Russian and Ukrainian officials used terms like “ecological disaster” and “terrorist act” to describe the torrent of water gushing through the broken dam and beginning to empty an upstream reservoir that is one of the world’s largest.

Ukrainian President Volodymyr Zelenskyy called it “the largest man-made environmental disaster in Europe in decades”. UN Secretary-General Antonio Guterres, ahead of an emergency Security Council meeting, called it “another devastating consequence of the Russian invasion of Ukraine”.

The environmental and humanitarian consequences quickly became clear as homes, streets and businesses flooded downstream and emergency crews began evacuations. Officials monitored water for cooling systems at the Zaporizhzhia nuclear plant, and authorities expressed concern about drinking water supplies in Ukrainian and Russian-controlled areas.

In the downstream city of Kherson, angry residents cursed as they tried to preserve their pets and belongings. A woman who gave her name only as Tetyana waded through thigh-deep water to reach her flooded house and rescue her dogs. They were standing on any dry surface they could find but one pregnant dog was missing. “It’s a nightmare,” she kept repeating, declining to give her full name.

Russian and Ukrainian authorities brought in trains and buses to move residents to safety. About 25,000 people in Russian-controlled areas and 17,000 in Ukrainian-held territory should be evacuated, Ukraine’s deputy chief prosecutor Viktoriia Lytvynova said on Ukrainian television. Neither side reported any deaths or injuries.

Russian news agency Tass quoted an unspecified Russian government official as saying the Kakhovka dam had “collapsed” due to damage. Photo / Maxar Technologies via AP

A satellite photo Tuesday morning by Planet Labs PBC analysed by the Associated Press showed more than 600 meters was missing from the wall of the 1950s-era dam.

The dam break, long feared by both sides, added a stunning new dimension to Russia’s war, 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 kilometres of front line in the east and south.

It was not immediately clear why either side might destroy the dam, and its collapse might have resulted from gradual degradation. Russian-controlled and Ukrainian-held lands were at risk.

Russian Defence Minister Sergei Shoigu charged that Ukraine destroyed the dam to prevent potential Russian attacks in the Kherson region after what he alleged was a failed Ukrainian counteroffensive. He claimed Ukraine had lost 3715 troops and 52 tanks since Sunday, and — in a rare acknowledgement of Russia’s own losses — said 71 Russian troops were killed and 210 wounded. Ukraine followed its standard practice of not commenting on its casualties.

Zelenskyy told reporters his government had information about Russia mining the dam last year, so “there may come a moment when an explosion occurs”. Other Ukrainian officials alleged Russia blew up the dam to hinder Kyiv’s counteroffensive, even though observers note that crossing the broad Dnieper would be extremely challenging. Other sectors of the front line are more likely avenues of attack, analysts say.

Nigel Gould-Davies, a senior fellow for Russia and Eurasia at the International Institute for Strategic Studies, said the alleged Russian destruction of the dam is “a profoundly defensive measure” showing “the lack of confidence in Russia’s longer-term prospects” in the war.

Experts have previously said the dam was in disrepair, which could also have led to the breach. David Helms, a retired American scientist who has monitored the reservoir since the war began, said in an email it wasn’t clear if the damage was deliberate or simple neglect from Russian forces occupying the facility.

But Helms also noted a Russian history of attacking dams.

Underscoring the global repercussions, wheat prices jumped 3 per cent after the collapse. It’s unclear whether the surge was due to a real threat of floodwaters destroying crops. Ukraine and Russia are key global suppliers of wheat, barley, sunflower oil and other food to Africa, the Middle East and parts of Asia.

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

Zelenskyy alleged Russian forces set off a blast inside the dam at 2.50am and said about 80 settlements were in danger.

A resident makes her way across a flooded road after the walls of the Kakhovka dam collapsed overnight, in Kherson, Ukraine. Photo / AP

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

White House National Security Council spokesman John Kirby said the US “cannot say conclusively what happened” and declined to say what impact the US believes it would have on Ukraine’s counteroffensive.

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

Andriy Yermak, the head of Ukraine’s President’s Office, posted video showing flooded streets in Russian-occupied Nova Kakhovka, a city with a prewar population of about 45,000.

Ukraine’s Interior Ministry urged residents of 10 villages on the Dnieper’s western bank and parts of the city of Kherson to gather essential documents and pets, turn off appliances, and leave.

The Russian-installed mayor of Nova Kakhovka, Vladimir Leontyev, said the city was being evacuated, and officials in other Ukrainian and Russian-controlled areas also began to move residents out.

The World Data Centre for Geoinformatics and Sustainable Development, a Ukrainian nongovernmental organisation, estimated nearly 100 villages and towns would be flooded.

Local residents try to ride their bikes along a flooded road after the Kakhovka dam blew overnight, in Kherson, Ukraine. Photo / AP

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

The UN’s International Atomic Energy Agency reported “no immediate risk to the safety of the plant”, whose reactors have been shut down for months but still need water for cooling. It said the dam’s reservoir level is falling by 5 centimetres an hour and could be depleted in a few days. The plant has alternative sources of water, including a large pond than can provide water “for some months”, according to the IAEA.

Ukrainian authorities have previously warned that the dam’s failure could unleash a volume of water estimated as nearly equivalent to that of the Great Salt Lake in the US.

Mykhailo Podolyak, a senior adviser to Zelenskyy, said “thousands of animals and ecosystems will be destroyed in the next few hours”.

German Chancellor Olaf Scholz and Nato Secretary-General Jens Stoltenberg 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 to the Black Sea and is crucial for the country’s drinking water and power supply and that of Russian-occupied Crimea.

Ukraine’s state hydropower generating company said the dam’s power station “cannot be restored”. Ukrhydroenergo also claimed Russia blew up the station from inside the engine room.

Ukraine and Russia have previously accused each other of attacking the dam.

- Vasilisa Stepanenko, Susie Blann, AP