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 U.S. 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 U.N. 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.

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