Ukraine says it is evacuating hundreds of people after part of the massive Nova Kakhovka dam was blown up by Russian forces.

“The scale of the destruction, the speed and volumes of water, and the likely areas of inundation are being clarified,” the South command of Ukraine’s Armed Forces wrote on Facebook on Tuesday (AEST).

Unverified footage of torrents of water spilling out of the dam in the Dnipro River is circulating widely on social media.

Other videos showed water surging through the remains of the dam with bystanders expressing their shock, sometimes in strong language.

The dam, 30 metres tall and 3.2 kilometres long, was built in 1956 on the Dnipro River as part of the Kakhovka hydroelectric power plant.

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

Up to 80 towns and villages downstream are risk of flood.

A senior Russia-installed official in the southern Ukraine region has acknowledged that overnight strikes destroyed parts of the Nova Kakhovka dam – despite earlier denying there was any damage.

Nova Kakhovka Mayor Vladimir Leontiev blamed Ukraine for the attack, calling it “a serious terrorist attack”, according to Russian state media TASS.

“Around 2am, there were a number of repeated strikes on the Kakhovka HPP (hydropower plant), which destroyed the ‘shonors’ [gate valves], as they are called in common parlance. As a result, water from the Kakhovka reservoir began to be spilled downstream uncontrollably,” Mr Leontiev said, according to TASS.

His admission came just hours after he told another Russian news agency, RIA Novosti, that claims the dam had collapsed were “nonsense.”

He said there was no need to evacuate Nova Kakhovka yet. However, Ukraine officials said on Tuesday afternoon (AEST) that evacuations had already begun in the region.

Ukraine President Vlodomyr Zelensky confirmed the attack in a tweet,= on Tuesday afternoon, blaming “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,” he wrote.

“Not a single metre should be left to them, because they use every metre 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 said all services were working, and he had convened a meeting of Ukraine’s National Security and Defence Council.

Ukraine’s military administration in the Kherson region urged residents to be ready to evacuate from villages on the right bank of the Dnipro.

“The water level is rising and everyone who is in the danger zone must: Turn off all electrical appliances; take documents and essentials; take care of loved ones and pets; follow the instructions of rescuers and policemen,” the administration said on its Telegram messaging channel.

Tuesday’s dam breach came as Russia said it had thwarted another major Ukrainian offensive in Donetsk, inflicting heavy losses, while Ukraine hailed progress in fighting in the east, although it was unclear if it marked the start of a long-anticipated Ukrainian counteroffensive.

On Monday, Russia said Ukrainian forces had begun a major offensive at the weekend in the southern part of the Donetsk region, which it had also thwarted.

Ukrainian officials have made no mention of any broad, significant new campaign although, in his nightly address on Monday, Mr Zelensky was enigmatic, hailing “the news we have been waiting for” and forward moves in Bakhmut in Donetsk.

However, the latest Russian defence ministry statement said Russian forces had 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 1500 troops.

Reuters could not independently verify the reports.

There was no immediate comment from Kyiv about Russia’s assertions.

Ukrainian Foreign Minister Dmytro Kuleba told Reuters on Monday Ukraine had enough weapons for a counteroffensive but declined to comment when asked whether it had begun.

In a promotional video released on Sunday and urging silence in regard to any military actions, Ukraine’s defence ministry said: “Plans like silence – the beginning will not be announced.”

The success or failure of a counteroffensive, 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.

-with AAP