In a statement the ministry wondered "why Russian soldiers are still sitting in muddy trenches rather than running to the aid of their comrades on both sides of the conflict. That would be far safer than confronting the Ukrainian army."

The Russian mercenary group Wagner, which had been fighting in Ukraine, has revolted against Moscow's armed forces and is bearing down on the capital. Ukraine, meanwhile, is pursuing an offensive to drive the remaining Russian troops from the country.

The Wagner group chief, Yevgeny Prigozhin, 62, released a series of messages from late Friday into Saturday, claiming that he and his mercenary troops had entered the southern Russian city of Rostov-on-Don and taken control of its military sites. 

Responding to the challenge in a televised address, Putin accused Prighozin, whose private army provided shock troops for Moscow's offensive in Ukraine, of a "stab in the back" that posed a threat to Russia's very survival.

The rebellion comes at a time when Russia is “fighting the toughest battle for its future,” Putin said, as Western governments heap sanctions on Moscow and arm Ukraine. Prigozhin's actions could have significant implications for the war.

The revolt by the Wagner mercenary group is considered the most serious challenge yet to shake the Kremlin chief's long rule, and Russia's most serious security crisis since the strongman Putin came to power in late 1999.

After Putin’s address, officials sought to reiterate their allegiance to the Kremlin and urged Prigozhin to back down.