The breach of Ukraine’s giant Nova Kakhovka dam is possibly the “most significant incident of damage to civilian infrastructure” since Russia invaded its neighbour 15 months ago, a UN boss says.

Tens of thousands of people are at risk from flooding along Ukraine’s Dnipro River, after a deliberate explosion breached a massive dam, with the United Nations aid chief warning of “grave and far-reaching consequences”.

As Russia and Ukraine pointed the blame at each other over the devastating collapse on Tuesday (local time), up to 70,000 people faced evacuation as floodwaters spread across a swathe of the war zone.

The incident has prompted fears of large-scale devastation. The Nova Kakhovka zoo said 300 animals died in the aftermath of the collapse, while Ukrainian troops have told of seeing Russian soldiers swept up in floodwaters and fleeing the east bank of the Dnipro River. Many Russian troops were killed or wounded, a Ukrainian officer said.

UN aid chief Martin Griffiths told the United Nations Security Council the dam breach “will have grave and far-reaching consequences for thousands of people in southern Ukraine on both sides of the front line through the loss of homes, food, safe water and livelihoods”.

“The sheer magnitude of the catastrophe will only become fully realised in the coming days,” he said.

Mr Griffiths said severe impacts were expected in Russian-occupied areas where humanitarian agencies were still struggling to gain access.

He highlighted the risks of fast-moving water on mine and explosive ordinance contamination, as it moved the projectiles to areas that had previously been deemed safe.

Mr Griffiths said the dam failure would also have an effect on electricity generation and the safety of the massive Zaporizhzhia nuclear power plant.

The UN nuclear watchdog has said the plant, which is upriver on the reservoir, should have enough water to cool its reactors for “some months” from a separate pond.

“The damage caused by the dam’s destruction means that life will become intolerably harder for those already suffering from the conflict,” Mr Griffiths said.

“The consequences of not being able to deliver assistance to the millions of people affected by the flooding in these areas are potentially catastrophic.”

Ukraine has accused Russia of committing a deliberate war crime in blowing up the Soviet-era Nova Kakhovka dam, which powered a hydroelectric station and supplied water for the nuclear plant and surrounding area.

The Kremlin has blamed Ukraine, saying it was trying to distract from the launch of a major counteroffensive Moscow says is faltering.

No deaths have been initially reported, but US spokesman John Kirby said the flooding had probably caused “many deaths”.

Ukrainian officials estimated about 42,000 people were at risk from the flooding, which was expected to peak on Wednesday.

In Kherson city, about 60 kilometres downstream from the dam, water levels rose by 3.5 metres on Tuesday, forcing residents to slog through water up to their knees to evacuate, carrying plastic bags full of possessions and small pets in carriers.

“Everything is submerged in water, all the furniture, the fridge, food, all flowers, everything is floating,” Oskana, 53, said when asked about her house.

“I do not know what to do.”

A CNN journalist has reported water rising rapidly, penetrating one block into Kherson in less than an hour. The flow of water was visibly increasing to the naked eye.

Buses, trains and private vehicles were marshalled to carry people to safety in about 80 communities threatened by flooding.

Oleksandr Prokudin, the head of the Kherson region military administration, said rescuing people and animals had become more difficult as time went on.

“If in the morning we could do it with cars, then with trucks. Now we see that big cars can no longer pass,” he said.

“The water has risen so much that we are now using boats. About eight boats of various types are currently working to evacuate people from the area.”

In Kherson, cracks of incoming artillery sent people trying to flee running for cover on Tuesday.

At night, Reuters reporters heard four incoming artillery blasts near a residential neighbourhood where civilians were evacuating.

Ukrainian President Volodymyr Zelensky said in a video address his prosecutors had already approached the International Criminal Court about the dam.

Earlier, he claimed on Telegram that Russian forces blew up the power plant from inside.

“Residents are sitting on the roofs of their homes waiting to be rescued,” Oleksiy Kuleba, a senior official on Mr Zelensky’s staff, said on Telegram.

“This is a Russian crime against people, nature and life itself.”

As Kyiv prepares for a long-awaited counteroffensive, some military analysts said the flooding could benefit Russia by slowing or limiting any potential Ukrainian advance along that part of the front line.

-with AAP