A new "revolution" could rock Russia if its stuttering war effort in Ukraine continues, the chief of private military group Wagner has said, in a scathing assessment of Moscow's military readiness that could further expose divisions in Russian President Vladimir Putin's military hierarchy.

Yevgeny Prigozhin said in an interview with Konstantin Dolgov, a pro-Russian blogger, that Moscow's troops are unprepared to resist forces loyal to Kyiv even when they enter Russian territory.

He also praised the capabilities of the Ukrainian army, and urged Moscow to escalate its war effort if it wants to avoid a long and costly conflict.

"I believe Ukrainians today are one of the strongest armies in the world," Prigozhin said. He called Kyiv's forces "highly organized, highly trained and their intelligence is on the highest level, they can operate any military system with equal success, a Soviet or a NATO one."

In recent days Moscow suffered embarrassment when a group of anti-Putin Russians entered the Belgorod region in an incursion that caused anger and confusion among Russia's influential military analysts. Asked about the incident, Prigozhin said Russian defense forces are "absolutely not ready to resist them in any shape or form."

"Here we are with Ukraine, that is our enemy, in the middle of the war, Russian Volunteer Corps groups come in effortlessly and go through (the border) in tanks and APCs without any repercussions and make their own videos if it," the Wagner chief vented.

Prigozhin has frequently criticized Russia's traditional military hierarchy as he sought to win a power struggle against military commanders to lead Putin's ground effort in eastern Ukraine. Earlier this month he blamed Russian defense chiefs for "tens of thousands" of Wagner casualties because they didn't have enough ammunition.

But his comments to Dolgov were alarmist even for the free-wheeling Putin ally. As he has frequently done, Prigozhin urged Moscow to step up its war in order to defeat Ukraine -- urging Putin to "declare a martial law and a new wave of mobilization."

He warned that if Russian losses continue to mount, "all these divisions can end in what is a revolution, just like in 1917."

"First the soldiers will stand up, and after that -- their loved ones will rise up. It is wrong to think that there are hundreds of them -- there are already tens of thousands of them -- relatives of those killed," he said. "And there will probably be hundreds of thousands -- we cannot avoid that."

Russian forces, primarily made up of Wagner troops, have labored for months over the capture of Bakhmut -- a city in Ukraine's east of relatively insignificant strategic value, where Russia has suffered vast losses -- and its larger ground campaign has been in stalemate since a string of successful Ukrainian counter-attacks last autumn.