The head of Russia's private Wagner mercenary group Yevgeny Prigozhin on Friday said Moscow's conventional army was leaving its positions near the eastern Ukraine hotspot town of Bakhmut.
His comments came just after Russia's Defense Ministry announced it had redeployed forces around to take up stronger defensive positions north of Bakhmut.
"This is not called regrouping, this is fleeing," Prigozhin said in an audio statement posted on social media.
In a separate video message, Prigozhin said the Defense Ministry units "simply went fleeing" from positions around north and south of the city.
"The flanks are failing. The front is collapsing," the Kremlin ally said.
He said Ukrainian forces had captured positions at a reservoir and on tactical heights around Bakhmut.
Kyiv's forces had also taken control of a road leading to Bakhmut from another smaller town further east, Chasiv Yar, which Russian forces had controlled.
AFP could not verify his claims.
Earlier on Friday Ukraine claimed to have retaken swathes of land near Bakhmut, amid speculations over a spring offensive from Kyiv.
Russia said, however, it had repelled Ukrainian attacks along a 95-kilometer (60-mile) stretch of the eastern front near Bakhmut.
"Attempts by the Defense Ministry in the information field to sugarcoat the situation — it's leading and will lead to a global tragedy for Russia," Prigozhin said.
"For this reason, we must stop lying immediately," he added.
A rivalry between Prigozhin and conventional army chiefs has come to the surface during the fight for Bakhmut, the longest and bloodiest of the conflict.
Prigozhin has repeatedly claimed that the Russian Defense Ministry was refusing to deliver ammunition and has accused its chiefs in vitriolic statements.