A huge Soviet-era dam on the Dnipro River that separates Russian and Ukrainian forces in southern Ukraine was breached on Tuesday, unleashing floodwater across the conflict zone and forcing villagers to flee.

Unverified videos on social media showed water surging through the remains of the dam with bystanders expressing their shock. Water levels raced up by meters in a matter of hours.

The Nova Kakhovka dam supplies water to the Crimean Peninsula and the Zaporizhzhia nuclear plant. The vast reservoir behind it is one of the main geographic features of southern Ukraine, 240 km long and up to 23 km wide. A swathe of countryside lies in the floodplain below.

The destruction of the dam happened when Ukraine had unleashed a long-awaited counteroffensive on Russian troops, and Russia claimed it thwarted a major Ukrainian attack in the Donetsk region over the weekend. On Tuesday, the defense ministry said a fresh Ukrainian assault had also been repelled.

Russia has controlled the dam since early in the conflict, although Ukrainian forces recaptured the northern side of the river last year. Both sides have long accused the other of planning to destroy it.

Following the dam burst, Ukraine accused Russia of destroying it. Meanwhile, Russian officials have given conflicting accounts, with some saying it was destroyed by Ukrainian shelling and others claiming it collapsed due to earlier damage.

Here's what we know about the breach so far: 

What's the significance of the dam?

Standing 30 meters tall and 3.2 kilometers long, the dam was built in 1956 on the Dnipro River as part of the Kakhovka hydroelectric power plant. The plant is in the city of Nova Kakhovka in the Kherson region, which is currently under Russian control. 

It supplies water to the Crimean Peninsula to the south and the Zaporizhzhia nuclear plant, Europe's largest nuclear plant, to the north. And it holds an estimated 18 cubic kilometers of water, about the same volume as the Great Salt Lake in the U.S. state of Utah. A dam burst could flood settlements below it and potentially wreck the canal system that irrigates much of southern Ukraine.

How did the dam breach?

It's still unclear whether the dam was artificially sabotaged or burst due to previous damage.

Ukraine, which commented first, said Russia was responsible, with President Volodymyr Zelensky saying Tuesday on the Telegram messaging app that Russian forces had destroyed the dam. He had urged the West to warn Russia not to blow up the dam in October, warning that it would flood a large area of southern Ukraine and is "an act to the use of weapons of mass destruction."

The south command of Ukraine's Armed Forces said on Tuesday that Russian forces blew up the dam. "The scale of the destruction, the speed and volumes of water, and the likely areas of inundation are being clarified," it said.

However, some Russian officials blamed Ukraine for the breach. Russia's Kherson administration said Ukraine shelled the dam at around 11 p.m. GMT, damaging the hydraulic valves.

"On Tuesday, June 6, the Ukrainian armed forces hit the Kakhovsky hydroelectric dam, as a result of which the upper part of the hydraulic structure was damaged," it said, adding that the dam was not totally destroyed.

Meanwhile, other Russian officials said no attack took place. Vladimir Rogov, a Russian official in Zaporizhzhia, said the dam collapsed due to earlier damage and water pressure.

What are the impacts?

With water levels surging higher, thousands of people are likely to be affected. Evacuations of civilians began on both sides of the front line. 

The water level in the town immediately adjacent to the breached dam could rise to 12 meters, and some 22,000 people living across 14 settlements in the southern Kherson region are at risk of flooding, according to Vladimir Leontyev, a Russian official in the Kherson region.

He asked residents in coastal settlements to be ready for evacuation, saying emergency and special services of the region are in full readiness and will provide all necessary assistance.

Ukrainian Prime Minister Denys Shmyhal said up to 80 settlements were at risk of flooding.

Russian officials on Tuesday 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 dam rupture. The Crimean peninsula is dependent on the canal's fresh water. It has sufficient water reserves for the moment, but falling water levels may cause water shortages in the region. Crimea said the level of risk would become clear in the coming days.

The UN nuclear watchdog, the International Atomic Energy Agency (IAEA), said there was no immediate nuclear safety risk at the Zaporizhzhia nuclear plant due to the dam failure but that it was monitoring the situation closely. The head of the plant also said there was no current threat to the station.

(With input from Reuters)