The water level around the Nova Kakhovka dam in the Kherson region is already up by five metres, with several downstream islands entirely flooded, according to Russian state-owned news agency RIA.

Local authorities fear that the water level could rise by up to 12 metres, according to Nova Kakhovka's Russian-installed mayor.

"The water continues to mount. An evacuation is being carried out of civilians from the adjacent flooded zones to preserve all lives ... There is no panic in the town," Vladimir Leontyev said in a video message on Telegram.

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Another Russian state-owned agency, TASS, said half the span of the 3.2km-long dam had been destroyed, and that the collapse of the remainder was ongoing.

Ukraine's state hydroelectric agency said that the plant had been "totally destroyed" after a blast in its engine room and could not be restored.

RIA also reported, citing the Kherson region's head, that 22,000 people in 14 settlements had been affected so far.

Both Ukrainian and Russian officials blamed each other for destroying the dam.

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 Facebook.

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

President Volodymyr Zelenskyy blamed "Russian terrorists" in a Telegram post, saying "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 wrote.

Andriy Yermak, the head of President Zelenskyy's administration, wrote on Telegram that the destruction was an attempt to "raise the stakes" in its full-scale invasion and stoke fears of a nuclear catastrophe.

"Today, the world must... understand that this is an attempt by terrorists to raise the stakes and scare everyone with a possible nuclear disaster," he wrote.

Russian forces blew up the dam "in a panic", Ukraine's military intelligence agency added.

"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.

The Ukrainian Interior Ministry called for residents of 10 villages on the Dnipro river's right bank and parts of the city of Kherson to gather "essential documents and pets, turn off appliances and leave".

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Oleksandr Prokudin, the head of the Kherson Regional Military Administration, said in a video posted to Telegram that "the Russian army has committed yet another act of terror", and warned that water will reach "critical levels" within five hours.

Meanwhile, TASS said the dam had been destroyed after being struck reportedly by firing from an Olkha multiple missile launcher, while Mr Leontyev said the strikes were "a very serious terrorist act" and Moscow-appointed authorities are "preparing for the worst consequences".

The Russian-installed governor of the Kherson region said that Kyiv had struck the dam to distract attention from what he said were the failures of the counteroffensive in the country's east.

In a video posted on Telegram, Vladimir Saldo also said that a major evacuation would not be necessary as a result of the dam breach.

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The dam was built in 1956 on the Dnipro river as part of the Kakhovka hydroelectric power plant and supplies water to the Crimean peninsula and to the Zaporizhzhia nuclear plant, which is also under Russian control.

Ukraine's state atomic agency said the dam's destruction posed a threat the nuclear plant but that the situation at the facility was currently under control.

"Water from the Kakhovka Reservoir is necessary for the station to receive power for turbine capacitors and safety systems of the ZNPP," Energoatom said in a statement on the Telegram messaging app.

"Right now the station's cooling pond is full: as of 8am, the water level is 16.6 meters, which is sufficient for the station's needs."

"Currently, the situation at the ZNPP is under control, Ukrainian personnel are monitoring all indicators," it said.

The UN 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".

The Russian-backed governor of Crimea said there is a risk that water levels in the North Crimea Canal, which carries fresh water to the peninsula from the Dnipro river, could fall after the blast.

In a statement on Telegram, Sergei Aksyonov said that Crimea had sufficient water reserves for the moment, and that the level of risk would become clear in the coming days.

Crimea is heavily dependent on water from the canal, which carries water from the Dnipro river, upstream of the dam. The canal was blocked by Ukraine after Russia annexed Crimea in 2014.

The blockade led to acute water shortages on the peninsula that ended only after Russian forces seized the canal in March 2022.

British Foreign Secretary James Cleverly said it was too early to give a meaningful assessment of what had occurred, but that it happened only because of Russia's invasion.

"I've heard reports of the explosion on the dam and the risk of flooding. It's too early to make any kind of meaningful assessment of the details," Mr Cleverly told Reuters while visiting Kyiv.

"But it's worth remembering that the only reason this is an issue at all is because of Russia's unprovoked full-scale invasion of Ukraine.

"We'll continue to assess the developing situation, but the best thing Russia could do now is withdraw their troops immediately."