Founder of Wagner private mercenary group Yevgeny Prigozhin speaks with servicemen during withdrawal of his forces from Bakhmut
Founder of Wagner private mercenary group Yevgeny Prigozhin speaks with servicemen during withdrawal of his forces from Bakhmut and handing over their positions to regular Russian troops, in the course of Russia-Ukraine conflict in an unidentified location, Russian-controlled Ukraine, in this still image taken from video released June 1, 2023. Reuters

Russian mercenary chief Yevgeny Prigozhin added fuel to his feud with the top brass on Thursday, accusing them of lying to President Vladimir Putin and the Russian people about the scale of Russian losses and setbacks in Ukraine.

Prigozhin, whose Wagner private militia spearheaded the Russian capture of the Ukrainian city of Bakhmut last month, is resisting an order for mercenary groups like his to sign contracts with the Defence Ministry before July 1.

Prigozhin portrays Wagner as Russia's most effective fighting force, and has enjoyed unusual freedom to publicly criticise Moscow - albeit not Putin, on whose support he and Wagner ultimately depend.

Yet the order to bring militias under Moscow's direct control suggested to some that he may have outlived his usefulness to Putin in challenging a military hierarchy that has failed to deliver the rapid victory he had hoped for.

In a series of emotional audio messages over two days, Prigozhin escalated his repeated criticism of Defence Minister Sergei Shoigu, a close Putin ally, and Valery Gerasimov, chief of the General Staff - by accusing them of hiding Russia's "very serious losses on the front" from Putin.

"Total trash is being put on the president's desk. Shoigu and Gerasimov have a simple approach. The lie must be monstrous for people to believe it. That is what they are doing," Prigozhin said in one message.

"It's all being hidden from everyone. Russia will wake up one day and learn that (Russian-annexed) Crimea has been handed over to the Ukrainians ...

"They are misleading the Russian people and if it keeps on like this we'll be left without the most important thing: Russia."

There was no immediate response from the Defence Ministry, which has ignored previous complaints, in public at least.

But it repeatedly says Russian troops have repelled all the attacks that Ukraine has launched in its two-week-old counteroffensive, inflicting a heavy cost in equipment and manpower while suffering only small losses itself.

Ukraine for its part has reported modest advances, which President Volodymyr Zelenskiy says have been "slower than desired". Reuters is unable to verify the battlefield accounts.