Ukraine on Tuesday accused Russian forces of blowing up a major dam and hydroelectric power station in a part of southern Ukraine that Moscow controls, sending water gushing from the breached facility and threatening what officials called an "ecological disaster" due to possible massive flooding.
Officials from both sides of the war ordered hundreds of thousands of residents downriver to evacuate.
Russian officials countered that the Kakhovka dam, on the Dnipro river, was damaged by 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 1,000-kilometres of frontline in the east and south of Ukraine.
Ukraine's nuclear operator Energoatom said in a Telegram statement that the blowing up of 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 wrote on Twitter that its experts were closely monitoring the situation at the plant, and there was "no immediate nuclear safety risk" at the facility.
Ukrainian authorities have previously warned that the dam's failure could unleash 18 million cubic metres (4.8 billion gallons) of water and flood Kherson and dozens of other areas where hundreds of thousands of people live.
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 to seven days.
A total collapse in the dam would wash away much of the left bank and a severe drop in the reservoir has the potential to deprive the nuclear plant of crucial cooling, as well as dry up the water supply in northern Crimea, 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 advisor 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".
Videos posted online began testifying to the spillover. One showed floodwaters inundating a long roadway; another showed a beaver scurrying for high ground from rising waters.
Zelenskyy called an emergency meeting to deal with the crisis, Ukrainian officials said.
The Ukrainian Interior Ministry called for residents of 10 villages on the Dnipro'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 controls five of the six dams along the Dnipro, which runs from its northern border with Belarus down to the Black Sea and is crucial for the entire country's drinking water and power supply.
Footage from what appeared to be a monitoring camera overlooking the dam that was circulating on social media purported to show a flash, explosion and breakage of the dam.
Oleksandr Prokudin, the head of the Kherson Regional Military Administration, said in a video posted to Telegram shortly before 7 am that "the Russian army has committed yet another act of terror", and warned that water will reach "critical levels" within five hours.
The Kakhovska dam was completely destroyed, Ukraine's state hydro power generating company wrote in a statement: "The station cannot be restored."
Ukrhydroenergo also claimed that Russia blew up the station from inside the engine room.
Leontyev, the Russian-appointed mayor, said on Tuesday that numerous 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.
Energoatom, the nuclear operator, wrote that the Kakhovka reservoir, where water levels are "rapidly decreasing", was necessary "for the plant to feed the turbine condensers and ZNPP safety systems", the statement said.
"Currently the station cooling pond is full: as of 8 am, the water level is at 16.6 metres, and this is enough for the needs of the station," it said.
Ukraine and Russia have previously accused each other of targeting the dam with attacks, and last October Zelenskyy predicted that Russia would destroy the dam in order to cause a flood.
Authorities, experts and residents have for months expressed concerns about water flows through -- and over -- the Kakhovka dam.
In February, water levels were so low that many feared a meltdown at the Zaporizhzhia nuclear plant, whose cooling systems are supplied with water from the Kakhovka reservoir held up by the dam.
By mid-May, after heavy rains and snow melt, water levels rose beyond normal levels, flooding nearby villages. Satellite images showed water washing over damaged sluice gates.