KYIV - Ukraine and Russia accused each other of blowing up a dam and causing widespread flooding in southern Ukraine, while Russia said it had thwarted another Ukrainian offensive in eastern Donetsk and inflicted heavy losses.

A vast Soviet-era dam in the Russian controlled part of southern Ukraine was blown up on Tuesday, unleashing a flood of water across the war zone, according to both Ukrainian and Russian forces.

There were immediate fears that the torrent of water could flood parts of the southern city of Kherson as well as 80 other settlements.

At least 22,000 people were believed to be at risk from the flood threat as millions of litres of water gushed out of the dam.

Evacuations of civilians were already underway after the “ecocide” attack amid reports that some parts of Kherson and villages were already under water.

One river was reported to be rising at 15cm an hour as Russia was being increasingly blamed for the dam breach as the expected widespread flooding could hamper a major Ukrainian counter-offensive.

The Russian-controlled Ukrainian town of Nova Kakhovka was under water by midday, the Russian state news agency TASS cited the Russian-installed mayor as saying on Tuesday, after water levels were said to have risen ten metres.

At least 300 homes were reported to have been evacuated, with similar emergency operations in a number of settlements in the Nova Kakhovka district, according to local officials.

Concerns were also raised that the huge surge in water could affect safety at the Zaporizhzhia Nuclear Power plant controlled by Russian forces.

There were reports that homes on the left bank of the giant Dnipro River were flooding, cattle drowning and that water levels could reach critical highs around midday.

Ukraine’s president Volodymyr Zelensky ordered an emergency meeting of his National Security and Defence Council and blamed the attack on the “Russian terrorists”.

Kherson’s regional head Oleksandr Prokudin also accused Moscow of committing “yet another act of terror” and warned that water levels in the Dnipro River will be “critical” within five hours.

He added that evacuation of civilians in at risk areas had already started on the Ukrainian-controlled right/western bank of the vast river, and urged similar action on the eastern Russian-seized side.

Britain blamed the dam breach on Vladimir Putin’s invasion, though stopped short at this stage of directly saying that the Russians had blown it up.

EU president Charles Michel said it was a “war crime,” adding: “We will hold Russia and its proxies accountable.”

Military experts said the flood of water could destroy some Russian defences but also hinder the long-awaited Ukrainian counter-offensive which some reports suggested was now underway.

Both sides blamed the other for destroying the dam.

Russian forces blew up the dam "in a panic", Ukraine's military intelligence agency said on Tuesday, amid reports that Kyiv forces could have used its road for a major counter-offensive.

"The occupiers blew up the dam of the Kakhovka Reservoir in a panic - this is an obvious act of terrorism and a war crime, which will be evidence in an international tribunal," it said in a statement on Telegram

Unverified videos on social media showed a series of intense explosions around the Kakhovka dam. A video posted by Ukrainian President Volodymyr Zelensky showed water pouring through the dam.

The dam, 30 metres (yards) tall and 3.2 km (2 miles) long, was built in 1956 on the Dnipro river as part of the Kakhovka hydroelectric power plant.

It holds an 18 km3 reservoir which also supplies water to the Crimean peninsula, annexed by Russia in 2014, and to the Zaporizhzhia nuclear plant, which is also under Russian control.

Ukraine’s military said that Russian forces blew up the dam.

“The Kakhovka (dam) was blown up by the Russian occupying forces,” the South command of Ukraine’s Armed Forces said on Tuesday on its Facebook page.

“The scale of the destruction, the speed and volumes of water, and the likely areas of inundation are being clarified.”

Russian news agencies said the dam, controlled by Russian forces, had been destroyed in shelling while a Russian-installed official said it was a terrorist attack - Russian shorthand for an attack by Ukraine.

The claim and counter claims could not be independently verified early on Tuesday morning.

Mr Zelensky tweeted: “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.

“Not a single meter should be left to them, because they use every meter for terror. It’s only Ukraine’s victory that will return security. And this victory will come. The terrorists will not be able to stop Ukraine with water, missiles or anything else.”

He added: “All services are working. I have convened the National Security and Defense Council. Please spread official and verified information only.”

In a statement, UkrHydroEnergo was reported to have said: “Russia blew up Kakhovka hydroelectric power plant in the early hours of 6th June. Its engine room has been destroyed completely. The station cannot be restored.

“By preliminary forecast, the reservoir is expected to run out within the next 4 days. As of 9 am on 6th June the water level in Kakhovka reservoir is decreasing rapidly, the evacuation of population from potentially flooded regions has begun.”

It was said to have added: “Uncontrolled decrease of water level is an additional threat for temporarily occupied Zaporizhzhia nuclear power plant. The water from Kakhovka reservoir is necessary so the plant receives power for turbine capacitors and safety systems of the ZNPP. Currently the station cooling pond is full. Ukrainian personnel at the plant is monitoring all indicators closely.”

On a visit to Kyiv, Britain’s Foreign Secretary James Cleverly said: “Clearly, there is an ongoing situation here.

“There is a risk to life because of the flood risk.

“The Ukrainian authorities will be working on this intensively.”

He added: “Of course, this is only happening because of Russia’s invasion of Ukraine.”

EU president Charles Michel tweeted: “Shocked by the unprecedented attack of the Nova Kakhovka dam. The destruction of civilian infrastructure clearly qualifies as a war crime - and we will hold Russia and its proxies accountable.”

Amid growing signs that a major Ukrainian offensive may have started, the Ministry of London tweeted: “Over the last 48 hours there has been a substantial increase in fighting along numerous sectors of the front, including those which have been relatively quiet for several months.”

In its latest intelligence update, the MoD also highlighted clashes between Putin’s Wagner Group “private army” and the regular Russian army.

“Concurrently, the feud between Wagner Group and the Russian MoD has reached an unprecedented level,” it said.

“For the first time, Wagner owner Yevgeny Prigozhin has claimed that the army has employed deliberate, lethal force against Wagner units. Following an altercation, Wagner has likely detained a Russian army brigade commander.

“Most of Wagner’s forces have now been withdrawn from Bakhmut. With Russia short of reserve units, the degree to which Wagner remains responsive to the MoD will be a key factor in the conflict over the coming weeks.”