Russia's president Vladimir Putin has vowed to crush what he called an armed mutiny in a televised address.
It comes after Yevgeny Prigojhin, head of the Wagner mercenary group said he wanted to oust Russia's military leadership.
The Russian army says it is carrying out 'combat measures' in the southern region of Voronezh. Further south, Prigozhin says his Wagner forces have taken control of key military sites in the city of Rostov-on-Don.
This includes the region's military headquarters - a key logistical base for Russia's invasion of Ukraine. But Prigozhin denies he's staging a coup.
As Russian authorities declare a counter-terrorist operation in Moscow, Wagner chief Yevgeny Prigozhin says his troops are prepared to die for their cause.