A video still shows an aerial view of the dam of the Kakhovka Hydroelectric Power Station after it was partially destroyed. AFP

Blasts at a Soviet-era dam in the Russian controlled part of southern Ukraine on Tuesday unleashed floodwaters across the war zone, according to both Ukrainian and Russian forces who blamed each other for blowing up the dam.

Ukraine accused Russian forces of blowing up a major dam and hydroelectric power station in a part of southern Ukraine they control, threatening a massive flood that could displace hundreds of thousands of people, and ordered residents downriver to evacuate.

A satellite image courtesy of Maxar shows a closer view of Nova Khakovka dam in south Ukraine on Monday. AFP

Russian news agency Tass quoted an unspecified Russian government official as saying the dam had "collapsed” due to damage.

The evacuation of areas near the Kakhovka region in southern Ukraine has begun, the governor of the Kherson region in Ukraine said on Tuesday.

"Within five hours the water will reach a critical level," regional governor Oleksandr Prokudin said on the Telegram channel as 6:45 a.m. (0345 GMT).

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Russia's TASS state news agency cited emergency services as saying that some 80 settlements in the area may be affected by the destruction of the Kakhovka dam.

Unverified videos on social media showed a series of intense explosions around the Kakhovka dam. Other videos showed water surging through the remains of the dam with bystanders expressing their shock, sometimes in strong language.

The dam, 30 metres (yards) tall and 3.2 km (2 miles) long and which holds water equal to the Great Salt Lake in the US state of Utah, was built in 1956 on the Dnipro river as part of the Kakhovka hydroelectric power plant.

This handout photo shows an overview of Nova Khakovka dam in south Ukraine on Monday. AFP

Ukrainian authorities have previously warned that the dam’s failure could unleash 18 million cubic metres (4.8 billion gallons) of water and flood Kherson and dozens of other areas where hundreds of thousands of people live. President Volodymyr Zelensky called an emergency meeting to deal with the crisis.

The nearby Russian-occupied Zaporizhzhia nuclear power plant could also be affected. Its cooling systems are supplied with water from the Kakhovka reservoir, held up by the dam. The International Atomic Energy Association wrote on Twitter that there was "no immediate nuclear safety risk at (the) plant.”

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

Russian-installed officials said there was no danger yet to the Zaporizhzhia nuclear plant, Europe's largest nuclear power plant, from the destruction of the dam. The nuclear power station gets its cooling water from the reservoir.

Ukraine's military said that Russian forces blew up the dam while Russian sources blamed Ukraine.