Russian President Vladimir Putin has delivered an emergency televised address on Saturday night (NZ time) after the chief of the Wagner Group Yevgeny Prigozhin launched an armed rebellion.
The feud between Prigozhin and the military top brass came to a head earlier on Saturday when he accused Russia's military of attacking a Wagner camp and killing a "huge amount" of his men. Prigozhin vowed to retaliate with force, crossed the border from Ukraine and seized the southern Russian city of Rostov-on-Don.
Russia's domestic intelligence service, the FSB, has opened a criminal case against Prigozhin, accusing him of "calling for an armed rebellion" and has urged his own mercenaries to detain him.
There are reports Prigozhin's forces are now heading towards Moscow, the seat of Russian power, with little opposition at this stage.
In a sign of how seriously Putin takes the situation, he has addressed the nation, comparing the rebellion to events in WWI and warning those on "path of treason" will be "punished".
"We will protect our people and state from any threats, including internal betrayal. What we're facing is exactly internal betrayal. Extraordinary ambitions and personal interests led to treason," he said, according to a translation.
"Any internal mutiny is a deadly threat to our state, to us as a nation. It's a strike against our nation, our people. And our actions to defend the fatherland from such a threat will be brutal. Anyone who consciously went on the path of betrayal, who prepared the armed mutiny, went on the path of blackmail and terrorist actions, will take an inevitable punishment. They will answer to the law and our people."
Putin said Russia will defend itself and added the armed forces have been "given the necessary orders".
"Extra anti-terrorist measures are now being implemented in Moscow, Moscow region, and a number of other regions. Decisive actions will be taken to stabilise the situation in Rostov-on-Don," he said, according to a translation.
"As a President of Russia and the Supreme Commander, as a citizen of Russia, I will do everything to defend the country, protect the Constitution, lives and safety, liberty of the citizens."
Russia's Ministry of Defense has appealed to Wagner forces to "safely return to their points of permanent deployment".
"You were tricked into Prigozhin's criminal adventure and participation in an armed rebellion," a statement said on its official Telegram channel.
"Many of your comrades from several squads have already realized their mistake by asking for help in ensuring the ability to safely return to their points of permanent deployment."
The ministry added it would "guarantee everyone's safety".
In an update on the situation, the UK's Ministry of Defence warned this represents "the most significant challenge to the Russian state in recent times".
"In Rostov-on-Don, Wagner has almost certainly occupied key security sites, including the HQ which runs Russia's military operations in Ukraine," the UK's Ministry of Defence said.
"Further, Wagner units are moving north through Vorenezh Oblast, almost certainly aiming to get to Moscow. With very limited evidence of fighting between Wagner and Russian security forces, some have likely remained passive, acquiescing to Wagner.
"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how the crisis plays out."