KYIV, Jan 11 (AFP) - Russia ordered its top general on Wednesday (Jan 11) to take charge of its faltering invasion of Ukraine in the biggest shake-up yet of its malfunctioning military command structure after months of battlefield setbacks.

It did so as Yevgeny Prigozhin, head of Russia's private military firm Wagner, said his forces had captured all of the eastern Ukrainian mining town of Soledar and killed about 500 Ukrainian soldiers after heavy fighting.

"I want to confirm the complete liberation and cleansing of the territory of Soledar from units of the Ukrainian army ... Ukrainian units that did not want to surrender were destroyed," he said in a statement.

"The whole city is littered with the corpses of Ukrainian soldiers," said Prigozhin, a close ally of Russian President Vladimir Putin.

Minutes earlier, Ukrainian President Volodymyr Zelenskyy mocked previous Wagner claims to have seized part of Soledar, saying fighting was still going on.

"The terrorist state and its propagandists are trying to pretend that part of our town of Soledar ... is some sort of a Russian possession," he said in a video address. "But fighting continues. The Donetsk theatre of operations is holding."

There was no immediate Ukrainian comment on Wagner's latest assertions.

In a separate statement on Facebook, the Ukrainian military general staff said Russian forces were suffering heavy losses as they tried to take Soledar and sever Ukrainian supply lines.

Russia has struggled to cement control over the salt-mining town, which would be Russia's most substantial gain since August after a series of retreats before Ukrainian counter-offensives in the east and south.

Wagner is among a number of semi-autonomous Russian forces whose high battlefield profile after more than 10 months of war has underlined the ineffectiveness of Russia's core military in an invasion it had expected to finish in days.

Russia's Defence Ministry said Defence Minister Sergei Shoigu had appointed Chief of the General Staff Valery Gerasimov as overall commander of forces for what Moscow calls its "special military operation" in Ukraine.

The move not only made Gerasimov directly accountable for the fate of the campaign but also in effect demoted General Sergei Surovikin, who is nicknamed "General Armageddon" by the Russian media for his reputed ruthlessness.

A Defence Ministry statement said the reshuffle was meant to improve contacts between different military branches and the "quality and effectiveness" of the command structure.

Mathieu Boulegue at think-tank Chatham House in London said that in shifting Gerasimov, Putin could be trying to increase "manual control" over management of the war and deflect criticism by pro-war ultra-nationalists inside and outside the Kremlin including Prigozhin.