Thousands of people are being evacuated downstream of a major dam which has collapsed in Russian-held Ukraine.
President Volodymyr Zelensky said 80 towns and villages may be flooded after the destruction of the dam at Nova Kakhovka, which he blamed on Russia.
Water is surging down the Dnipro river, and is said to pose a catastrophic flooding risk to the city of Kherson.
Russia has denied destroying the dam – which it controls – instead blaming Ukrainian shelling.
Neither Ukraine nor Russia’s claim has been verified by the BBC.
The Kakhovka dam, downstream from the huge Kakhovka reservoir, is crucial to the region.
It provides water to farmers and residents, as well as to the Zaporizhzhia nuclear power plant. It is also a vital channel carrying water south to Russian-occupied Crimea.
Ukraine’s state-owned hydropower plants administrator Ukrhydroenergo warned that the peak of a water spill downstream from the emptying reservoir was expected on Wednesday morning.
It said this would be followed by a period of “stabilisation”, with the water expected to rapidly recede in four to five days.
There are concerns about the Zaporizhzhia nuclear power plant – Europe’s largest – which uses reservoir water for cooling.
The situation there is said to be under control and there is “no immediate nuclear safety risk” for the plant, according to the International Atomic Energy Agency (IAEA).
Video footage shows a torrent of floodwater gushing through a breach in the dam. Several towns are already flooded, while people in areas further downstream have been forced to flee by bus and train.
Abound 40,000 people need to be evacuated, Deputy Prosecutor-General Viktoriya Lytvynova said on Ukrainian television – 17,000 people in Ukraine-controlled territory west of the Dnipro River and 25,000 on the Russian-controlled east.
Also speaking on Ukrainian television, Interior Minister Ihor Klymenko said about 1,000 people had been evacuated so far and 24 settlements had been flooded.
He accused Russia of shelling the southern region of Kherson, from where people were being evacuated, and issued a warning about the dangers posed by mines being exposed by the rising water levels.
One local resident Andriy, who lives close to the dam – which was seized by Russian forces shortly after Moscow launched its full-scale invasion in February 2022 – said he believed Russia wanted to “drown” his city.
In the Ukraine-controlled city of Kherson, a woman called Lyudmyla – who was loading her belongings including a washing machine onto a trailer that was attached to an old car – said: “We’re afraid of flooding. We’re taking our things a little higher up.”
She called for Russian forces to be “kicked out of here… they’re shooting at us. They’re flooding us or doing something else”.
Another resident of the city, Serhiy, said he feared “everything is going to die here”.
“All the living creatures, and people will be flooded out,” he said, gesturing at nearby houses and gardens.
On the Russian-seized riverbank of Nova Kakhovka, the Moscow-installed mayor Vladimir Leontyev said the city was underwater and 900 people had been evacuated.
He said 53 evacuation buses were being sent by the authorities to take people from the city and two other settlements nearby to safety.
Water levels had risen to over 11m (36ft) and some residents had been taken to hospital, he added.
The small town of Oleshky was also heavily flooded, Kremlin-appointed officials said.
The Kazkova Dibrova zoo on the Russian-held riverbank had been completely flooded and all 300 animals were dead, it said in a post on its Facebook page.
It is not yet clear what caused the breach in the dam in the early hours of Tuesday, but Ukraine’s military intelligence has accused Russia of deliberately blowing it up.
This seems plausible, as Moscow may have feared that Ukrainian forces would use the road over the dam to advance into Russian-held territory, as part of their counter-offensive.
For Russia, anxious to defend conquered territory in southern Ukraine, the dam represented an obvious problem.
Just as Ukrainian forces attacked road and rail bridges further downstream last autumn in a successful effort to isolate Russian forces in and around Kherson, Russia may have decided to destroy the dam to hold up Ukraine’s counter-offensive, which it fears could come from multiple directions.
However, a Russian official claims Ukraine carried out the attack on the dam to detract from what they said were the failures of its counter-offensive and to deprive Crimea – Ukraine’s southern peninsula illegally annexed by Russian in 2014 – of fresh water.
A major Ukrainian push has long been expected. Kyiv has said it would not give advance warning of its start but a recent increase in military activity is being seen as a fresh sign that the counter-offensive may have begun.
On Tuesday evening, President Zelensky said the dam destruction would not stop Ukraine. “We will still liberate all our land,” he said in a video address.
Earlier in the day, Mr Zelensky held an urgent meeting of the country’s security and defence council to discuss the issue.
On Monday, Ukraine’s deputy defence minister Hanna Maliar said Ukrainian forces had advanced around the “epicentre of hostilities” in Bakhmut, but did not say whether the counter-offensive had begun.
Bakhmut has for months been at the heart of fierce fighting. It has little strategic value – but is important symbolically both for Kyiv and Moscow.
Yuri Sak, an adviser to Ukraine’s ministry of defence, told the BBC Radio 4’s Today programme that phone intercepts suggest Russia wants to target more dams.
“They’re actually calling to blow up more dams on the Dnipro river,” he said.
Ukraine has branded the attack on the dam “ecocide” and said that 150 tonnes of engine oil has spilled into the Dnipro river.
Ukrhydroenergo said a power station linked to the dam had been “completely destroyed… the hydraulic structure is being washed away”.
World leaders have laid the blame for the blast at Russia’s door, with some calling it a war crime.
UK Prime Minister Rishi Sunak said that if Russia was found to be responsible for the collapse of the dam it would “demonstrate the new lows that we will have seen from Russian aggression”.
The head of Nato, Jens Stoltenberg, said the destruction of the dam demonstrated once again the brutality of Russia’s war in Ukraine, while Charles Michel, the president of the European Council, said he was “shocked by the unprecedented attack”.
The Geneva Conventions explicitly ban targeting dams in war due to the danger it poses to civilians.
