KHERSON, Ukraine — Thousands of people escaped inundated homes in southern Ukraine on Wednesday, including many rescued from rooftops, a day after the destruction of the Kakhovka dam gave rise to another humanitarian disaster along the front lines of the 15-month war.
Floodwaters engulfed streets and houses and sent residents fleeing with what meager belongings they could carry from dozens of communities on both sides of the Dnipro River, which divides the warring armies in much of southern Ukraine.
As the debris-choked waters began to peak on Wednesday, reports indicated that some 4,000 people had been evacuated in Russian- and Ukrainian-controlled areas, according to officials on both sides, a fraction of the roughly 41,000 residents who Ukraine estimates were at risk from the flooding. The U.S. State Department estimated that about 20,000 people would have to be resettled.
It was still unclear what caused the dam’s failure. Experts said a deliberate explosion inside the dam, which has been under Russian control since early in the war, most likely caused the massive structure of steel-reinforced concrete to crumble.
President Volodymyr Zelensky of Ukraine said Russian forces, who have consistently used the destruction of Ukrainian infrastructure as a tactic of war, had blown up the dam to “use the flood as a weapon.” Russian officials blamed Ukrainian shelling for damaging the facility, but experts said that was very unlikely to cause it to collapse. A State Department spokesman, Vedant Patel, said the United States could not say who was behind the dam’s failure.
In calls with Mr. Zelensky and Russian President Vladimir V. Putin, Turkey’s president, Recep Tayyip Erdogan, called for a “comprehensive investigation” by a commission with U.N. representatives, as well as Russian and Ukrainian experts. Mr. Erdogan said the inquiry should be done “in a way that leaves no room for suspicion.”
There were still no confirmed reports of deaths, and the scale of the disaster, which drained a giant reservoir used for drinking water and irrigation, was only beginning to come into focus. At least seven people were reported missing in the flooding, the Russian state news agency Tass reported, citing Vladimir Leontiev, the Russian-installed mayor of Nova Kakhovka. On the Ukrainian side, three people were unaccounted for, the National Police said.
Mr. Zelensky said that hundreds of thousands of people were “without normal access to drinking water” and that the emergency services were working to rush potable water to Ukrainian-controlled areas.
In the Ukrainian-held city of Kherson on the west bank, rescuers completely evacuated a neighborhood submerged in fetid floodwaters, venturing out in boats to pull people from roofs and the upper floors of homes. The river peaked at about 10 feet above normal in Kherson, and Ukrhydroenergo, the country’s main hydroelectric power company, said it would begin to recede in the coming days. Mykolaiv, a Black Sea port city that was already under strain as a hub for people fleeing fighting, was offering shelter to evacuees.
Information about areas in the Russian-occupied east bank was difficult to obtain, but state television broadcast images of inundated villages, and Russian-appointed officials said about 1,500 people there had been evacuated.
Vladimir Saldo, the Russian-installed occupation governor in the Kherson region, said that 48 temporary housing facilities with 2,700 beds had been set up, in part with the help of Russia’s emergencies ministry. He declared a state of emergency and listed 35 towns that had been affected by the flooding on the Russian-held side of the Dnipro, including places where the water reached the roofs of buildings.
Stoking fears that Russia was continuing a practice that has prompted the International Criminal Court to issue an arrest warrant against Mr. Putin and one of his top aides, Mr. Saldo said occupation authorities were taking children from the flooded settlements below the dam and sending them to holiday camps in other parts of the Kherson region or in Crimea. Over the past year, Ukraine and rights officials have condemned the displacement or forcible transfer of Ukrainian children and called it a war crime.
The environmental toll of the disaster was also becoming clear. Ukraine’s agriculture ministry warned that the dam’s destruction cut off water to hundreds of thousands of acres of farmland, turning some of the country’s most productive grain fields “into deserts as early as next year.” The dam held back a body of water the size of the Great Salt Lake in Utah.
Ukraine’s Health Ministry said thousands of fish had died, and environmental groups warned that the drop in the reservoir’s water levels would make it difficult for fish eggs to hatch and replenish populations.
Ukrainian officials also said roughly 150 tons of machine oil were released from an engine room in the dam, sending toxic waters downstream. Another 300 tons of the oil were still at risk of leaking into the river. Environmental groups warned of pesticides, fuels and other toxins being washed into Dnipro.
The dam’s destruction could also risk diverting attention, resources and personnel from a long-planned Ukrainian counteroffensive that U.S. officials said may have begun this week. Flood-affected communities are calling for large amounts of fuel, water and vehicles — all components that are also essential for military operations — while national guard soldiers are helping with disaster relief.
Fighting continued on Wednesday on the front lines of eastern Ukraine, with Ukraine bombarding Russian positions and Russia attacking Kherson, even as the flooded city tried to evacuate residents.
NATO Secretary General Jens Stoltenberg warned on Wednesday not to focus too heavily on the Kremlin’s battlefield blunders so far.
“What they lack in quality — they have bad morale, bad equipment, bad training, bad leadership, bad logistics — they compensate in quantity, and quantity has a quality in itself as the generals keep on telling us,” he said.
Andrew E. Kramer reported from Kherson, Ukraine, Paul Sonne from Berlin and Victoria Kim from Seoul.