A wall of a major Ukrainian dam collapsed on Tuesday, triggering floods in the region and displacing people from their homes.
The flooding of the region has also led to fears around Ukraine's Zaporizhzhia Nuclear Power Plant, which is the largest nuclear plant in Europe.
Ukraine has accused Russia of blowing up the Kakhovka dam and hydroelectric power station. Russia has blamed Ukrainian bombardment in the contested area for the breach in the dam.
Footage from the region shows large swathes of land covered with water.
It was not immediately clear which side would benefit from the breach in the dam as both the Ukraine- and Russia-held territories are at risk from the flooding. Authorities expressed concern about supplies of drinking water to the south in Crimea, which Russia illegally annexed in 2014.
Around 38,000 affected by flooding
The potentially far-reaching environmental and social consequences of the Kakhovka dam breach quickly became clear as homes, streets and businesses flooded downstream and emergency crews began evacuations.
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 World Data Center for Geoinformatics and Sustainable Development, a Ukrainian nongovernmental organisation, 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.
Environmental consequences of dam breach
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 organisation 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.
Ukraine and Russia trade allegations
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 am (2350 GMT Monday) 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 tonnes of oil escaped from the dam machinery and that another 300 metric tonnes 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.
Which side could gain from the dam breach?
It was not clear which side would gain from the dam breach as the flooding would affect people on both the sodes. However, there could be one benefit to the Russians as the flooding could further delay the Ukrainian counter-offensives, which has now been anticipated for several weeks.
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 kilometres (621 miles) of front line in the east and south.
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.
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.
How does the dam collapse affect Zaporizhzhia nuclear plant?
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 UN'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 five centimetres (two 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 that can provide water "for some months", the statement said.
The damaged power station can't be stored
The Russian and Ukrainian authorities said that the damaged power station at the dam cannot be restored.
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.
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.
(With AP inputs)
