Factbox-Is the Kakhovka dam in Ukraine about to be blown?

A satellite image shows a view of the location of the Kakhovka dam and the surrounding region in Kherson Oblast

(Reuters) - Russia and Ukraine have accused each other of planning to blow up the Kakhovka hydro-electric dam on the Dnipro River, a step that would unleash a devastating flood across a large area of southern Ukraine.

What is the Kakhovka dam, is it about to be blown and what impact would that have?

SIGNIFICANCE OF THE DAM

* The dam, 30 metres (yards) tall and 3.2 km (2 miles) long, was built in 1956 on the Dnipro river as part of the Kakhovka hydroelectric power plant.

* It holds an 18 km3 reservoir which also supplies water to the Crimean peninsula, annexed by Russia in 2014, and to the Zaporizhzhia nuclear plant, which is also under Russian control.

* The volume of water in the reservoir is about equal to the Great Salt Lake in the U.S. state of Utah.

* Blowing the Soviet-era dam, which is controlled by Russia, would unleash a wall of devastating floodwater across much of the Kherson region which Russia last month proclaimed as annexed in the face of a Ukrainian advance.

* Destroying the Kakhovka hydro-electric power plant would also add to Ukraine's energy woes after weeks of Russian missile strikes aimed at generation and grid facilities which Kyiv said have damaged a third of its country-wide power network.

ALLEGATIONS

* Sergei Surovikin, the commander of Russian forces in Ukraine, said on Tuesday he had information that Ukrainian forces were preparing a massive strike on the dam and had already used U.S.-supplied HIMARS missiles of a major strike, he said, could be a disaster.

"We have information on the possibility of the Kyiv regime using prohibited methods of war in the area of the city of Kherson, on the preparation by Kyiv of a massive missile strike on the Kakhovka hydro-electric dam," Surovikin said.

Ukrainian officials said the allegation was a sign that Moscow planned to attack the dam and blame Kyiv.

* Ukrainian President Volodymyr Zelenskiy said on Thursday that Russia had mined the dam and was preparing to blow it, a step he compared to the use of weapons of mass destruction.

"I informed the Europeans today, during the meeting of the European Council, about the next terrorist attack, which Russia is preparing for at the Kakhovka hydroelectric power plant," he said. "Destroying the dam would mean a large-scale disaster."

Blowing the dam, he said, would also destroy the water supply to Crimea and thus show that Russia had accepted that it could not hold onto the peninsula.

Kirill Stremousov, the Russian-installed deputy head of the annexed Kherson region, said Kyiv's allegations that Russia had mined the dam were false.

(Reporting by Reuters; editing by Guy Faulconbridge and Philippa Fletcher)