A torrent of water burst through a massive dam on the Dnipro River that separates Russian and Ukrainian forces in southern Ukraine on Tuesday, flooding a swathe of the war zone and forcing villagers to flee.

Ukraine accused Russia of blowing up the Nova Kakhovka dam in a deliberate war crime. The Kremlin said it was Ukraine that had sabotaged the dam, to distract attention from the launch of a major counteroffensive Moscow says is faltering. Some Russian-installed officials said the dam had collapsed on its own.

Neither side offered immediate public evidence of who was to blame. The Geneva Conventions explicitly ban targeting dams in war because of the danger to civilians.

By mid-morning in the city of Kherson in Ukrainian government-controlled territory downstream from the dam, a pier on a tributary of the Dnipro had already been submerged.

Lidia Zubova, 67, waiting for a train out of the city after abandoning her inundated village of Antonivka, told Reuters: “Our local school and stadium downtown were flooded … The road was completely flooded, our bus got stuck.”

Ukrainian police released a video of an officer carrying an elderly woman to safety and others rescuing dogs in villages being evacuated as the waters rose. Interior Minister Ihor Klymenko accused Russia of shelling areas from where people were being evacuated and said two police officers were wounded.

On the Russian-controlled bank of the Dnipro, the Moscow-installed mayor of Nova Kakhovka said water levels had risen to 11 metres. Residents reached by telephone there told Reuters that some had decided to stay despite being ordered out by occupying Russians.

“They say they are ready to shoot without warning,” said one man, Hlib, describing encounters with Russian troops. “If you come a metre closer than allowed, they immediately start yelling obscenities. We’re still allowed to go to the store, but we don’t know what orders will be given next.”

Yevheniya, a female resident, said the water was up to the knees of the Russian soldiers walking the main street in high rubber boots. “If you try to go somewhere they don’t allow, they immediately point their machine guns at you,” she said. “More and more water is coming every hour. It’s very dirty.”

The Kazkova Dibrova Zoo on the Russian-held riverbank was completely flooded and all 300 animals were dead, a representative said via the zoo’s Facebook account.

The small town of Oleshky, on the Russian-controlled bank of the Dnipro, was almost completely flooded, a Russian-appointed regional official said on Tuesday.

“Evacuation … is possible only using special equipment,” Andrei Alexeyenko, chairman of the Russian-appointed government of Ukraine’s Kherson province, said on Telegram.

The dam supplies water to a wide area of southern Ukrainian farmland, including the Russian-occupied Crimean peninsula, as well as cooling the Russian-held Zaporizhzhia nuclear plant.

The vast reservoir behind the dam is one of the main geographic features of southern Ukraine, 240 kilometres long and up to 23km wide.

An expanse of countryside lies in the flood plain below, with low-lying villages on the Russian-held southern bank particularly vulnerable.

Ukraine and Russia have both asked the UN Security Council to meet to discuss the dam. Ukraine accused Russia of an “ecological and technological act of terrorism”, while Russia described it as an “act of sabotage carried out by Ukraine”, according to the requests seen by Reuters.

Ukraine Environment Minister Ruslan Strilets planned a press briefing at 1700 GMT.

Poised for counteroffensive

The dam’s destruction raised fears of a new humanitarian disaster in the centre of the war zone and transformed the front lines just as Ukraine prepares a long-awaited counteroffensive to drive Russian troops from its territory.

Russian Defence Minister Sergei Shoigu said his forces had thwarted the first three days of the offensive in battles that had left more than 3,700 Ukrainian soldiers dead or wounded.

Ukraine dismissed the Russian statements as lies but gave no details on the attacks. President Volodymyr Zelenskiy said in an interview published on Saturday that Ukraine was poised to unleash its much-heralded major counteroffensive, using newly supplied Western battle tanks and armoured vehicles.

Russia has controlled the dam since early in its 15-month-old invasion, although Ukrainian forces recaptured the Dnipro’s northern bank last year. Both sides had long accused the other of plotting to destroy the dam.

“Russian terrorists. The destruction of the Kakhovka hydroelectric power plant dam only confirms for the whole world that they must be expelled from every corner of Ukrainian land,” Zelenskiy wrote on the Telegram messaging app.

Russians had “carried out an internal detonation of the structures” of the dam, Zelenskiy said. “About 80 settlements are in the zone of flooding.”

Nato Secretary-General Jens Stoltenberg called it “an outrageous act, which demonstrates once again the brutality of Russia’s war in Ukraine”.

Kremlin spokesman Dmitry Peskov blamed “deliberate sabotage by the Ukrainian side”.

Russian-installed officials had earlier given conflicting accounts, some saying the dam had been hit by Ukrainian missiles overnight, others saying it had burst on its own due to earlier damage.

The UN nuclear watchdog said the Zaporizhzhia power plant, upriver on the reservoir’s Russian-held bank, should have enough water to cool its reactors for “some months” from a separate pond, even as the huge reservoir drains out.

Video showed water surging through the remains of the dam — which is 30m tall and 3.2km long.

Some 22,000 people living across 14 settlements in the Kherson region are at risk of flooding, Russia’s RIA news agency quoted the Moscow-installed head of the region as saying.