Millions of litres of water burst through a gaping hole in a Russian-controlled dam on Tuesday, flooding a swathe of the war zone in southern Ukraine, threatening scores of villages and cutting off water supplies.
Ukrainian and Russian forces blamed each other for the breach.
The Nova Kakhovka dam, which holds water equal to that in the Great Salt Lake in the US state of Utah, supplies water to Ukraine's Crimean peninsula, annexed by Russia in 2014, and to the Zaporizhzhia nuclear plant, also under Russian control.
The UN nuclear watchdog, the International Atomic Energy Agency, said on Twitter it was closely monitoring the situation but that there was "no immediate nuclear safety risk at (the) plant" which is also in southern Ukraine.
However, Ukraine's state atomic power agency Energoatom said the water level of the Kakhovka Reservoir was rapidly lowering, posing an "additional threat" to the facility, Europe's largest nuclear power plant.
Some 22,000 people living across 14 settlements in Ukraine's southern Kherson region are at risk of flooding, Russia's RIA news agency quoted the Moscow-installed head of the region as saying. Kherson is one of five regions, including Crimea, that Moscow claims to have annexed.
Unverified videos on social media showed water surging through the remains of the dam with bystanders expressing their shock. Water levels raced up by metres in a matter of hours.
A Russian-installed official in the town of Nova Kakhovka said on Tuesday residents of around 300 houses had been evacuated, state-owned news agency TASS reported. He said it would likely be impossible to repair the dam.
COUNTER-OFFENSIVE
The dam breach came as Ukraine prepares to launch its long-awaited counter-offensive to drive Russian forces from territory they have seized during more than 15 months of fighting.
Russia said it had thwarted another Ukrainian offensive in eastern Donetsk and inflicted heavy losses. It also launched a fresh wave of overnight air strikes on Kyiv. Ukraine said its air defence systems had downed more than 20 cruise missiles on their approach to the capital.
Reuters could not independently verify the reports and it was unclear whether any of the latest fighting marked the beginning of Ukraine's long-anticipated counter-offensive.
The Southern Command of Ukraine's military accused Russian forces of blowing up the Nova Kakhovka dam, which is 30 metres (yards) tall and 3.2 km (2 miles) long. It was built in 1956 on the Dnipro river.
"The scale of the destruction, the speed and volumes of water, and the likely areas of inundation are being clarified," the Ukrainian military said on Facebook.
Ukraine's military intelligence agency later said on Telegram that Russian forces had blown up the dam "in a panic", in what it said was "an obvious act of terrorism and a war crime, which will be evidence in an international tribunal".
Russian news agencies said the dam had been destroyed in shelling while the mayor of Russia-controlled Nova Kahhovka city was quoted as blaming an act of terrorism - Russian shorthand for an attack by Ukraine.
The Russian installed head of the Kherson region said evacuation near the dam had begun and that water would reach critical levels within five hours.
The Kakhovka Hydroelectric Power Plant has been "totally destroyed" and cannot be restored after a detonation inside the engine room, Ukraine's state hydroelectric company said.
Ukraine's President Volodymyr Zelenskiy will hold an emergency meeting over the dam blast, Oleksiy Danilov, secretary of Ukraine's National Security and Defence Council, said on Twitter on Tuesday.
UKRAINIAN ATTACKS
Russian President Vladimir Putin sent troops into Ukraine on Feb. 24 last year in what the Kremlin expected to be a swift operation, but its forces suffered a series of defeats and regrouped in the country's east.
Tens of thousands of Russian troops dug in over the winter, besieging Bakhmut for months and bracing for an expected Ukrainian counter-attack to try to cut Russia's so-called land bridge to the Crimean Peninsula.
Ukrainian officials have made no mention of any broad, significant new campaign, although in his nightly address on Monday, Ukrainian President Volodymyr Zelenskiy was enigmatic, hailing "the news we have been waiting for" and forward moves in Bakhmut in Donetsk.
Russia says it thwarted a major Ukrainian attack in the Donetsk region over the weekend and on Tuesday the defence ministry said a fresh Ukrainian assault had also been repelled.
Russian forces inflicted huge personnel losses on attacking Ukrainian forces and destroyed 28 tanks, including eight Leopard main battle tanks and 109 armoured vehicles, it said. Total Ukrainian losses amounted to 1,500 troops.
There was no immediate comment from Kyiv about Russia's assertions. Russia and Ukraine have often made claims of inflicting heavy human losses on each other which could not be verified.
Writing on Telegram, Russia's Wagner militia leader Yevgeny Prigozhin said Moscow's claims of huge Ukrainian losses were "simply wild and absurd science fiction."
The Washington Post reported that some US officials thought Ukraine's counter-offensive was underway, but White House national security spokesperson John Kirby declined to comment on whether this was the case.
"I'm not going to be talking for the Ukrainian military," he told a briefing, adding that the United States had done "everything we could ... to make sure that they had all the equipment, the training, the capabilities to be successful."
The success or failure of a counter-offensive, expected to be waged with billions of dollars worth of advanced Western weaponry, is likely to influence the shape of future Western diplomatic and military support for Ukraine.
In its evening report on Monday, Ukraine's General Staff made no mention of any large-scale offensive, nor did it suggest any deviation from the usual tempo or scope of fighting along front lines that have not changed significantly for months.
