The boss of Russia’s Wagner group, Yevgeny Prigozhin, says its mercenaries have seized “all military facilities” in the southern city of Rostov-on-Don.
They are also said to have deployed in Voronezh, further north, following his threat to march on Moscow to topple Defence Minister Sergei Shoigu.
There have also been sightings of Wagner vehicles on the highway linking Voronezh to Moscow.
President Vladimir Putin called the rebellion “a stab in the back”.
He pledged to punish those who had “betrayed” Russia.
Tighter security has been introduced in many regions, including Moscow.
The Wagner Group is a private army of mercenaries that has been fighting alongside the regular Russian army in Ukraine.
Tension has been growing between them over how the war has been fought, with Prigozhin launching vocal criticisms of Russia’s military leadership in recent months.
The dramatic escalation came after Prigozhin accused Russia’s military of hitting his group’s base in Ukraine – a claim denied by Moscow.
Prigozhin claims this is not a military coup, but his aim seems to be to topple Russia’s military leadership, reports the BBC’s Russia editor Steve Rosenberg in Moscow.
In an emergency address, President Putin said that Russia was facing “treason” – without mentioning Prigozhin, a former ally, by name.
“All those who consciously stood on the path of betrayal, who prepared an armed rebellion, stood on the path of blackmail and terrorist methods, will suffer inevitable punishment, before the law and before our people,” Putin vowed.
He also acknowledged the situation in Rostov-on-Don was difficult.
In his response – the first time Prigozhin had criticised the Russian leader directly – the Wagner boss said his troops were “patriots of our motherland” and President Putin was “deeply wrong” to talk of betrayal.
In a video posted on his Telegram channel earlier on Saturday, Prigozhin said his fighters had entered Rostov-on-Don after crossing the border from Ukraine – and his men would destroy anyone who stood in their way.
He said his forces had shot down a Russian military helicopter that “opened fire on a civilian convoy” – no location was given and that assertion could not be immediately verified.
He has also said that his fighters have been fired on by artillery and helicopters, although he did not specify where those incidents took place.
A “counter-terrorism operation” has been launched in the Voronezh region, north of Rostov and en route to Moscow, where Wagner forces are also said to be in control of some military facilities.
The governor of the Lipetsk region, north of Voronezh, has also confirmed that Wagner vehicles are moving through the area.
Although it is difficult to verify information on the ground, the information suggests that the Wagner vehicles are moving towards Moscow. However, it is unclear what their exact plans are.
Prigozhin and President Putin used to have a close relationship.
But the latest challenge is a headache for the Russian leader – on top of the fact that the Russian invasion of Ukraine has not been going to plan – our Moscow correspondent says, and Mr Putin has now decided to say “enough is enough”.
