Updated 3pm

• Key military base in Rostov taken over by Wagner mercenaries
• Putin describes move as 'high treason' and vows punishment
• Wagner convoy seen en route to Moscow
• Capital placed under "anti-terrorist operation"

President Vladimir Putin warned Saturday that treason against his rule threatens Russia with civil war, as Moscow's military launched an operation to tackle a revolt by the Wagner mercenary group. 

Wagner chief Yevgeny Prigozhin, once a close Putin ally, said his troops had taken control of the military command centre and bases in the southern city of Rostov-on-Don, the nerve centre of Russia's offensive in Ukraine, and vowed to topple Moscow's top military leaders.

Further north, on Wagner's possible route towards Moscow, the governor of Russia's Voronezh region said the armed forces had launched a "counter-terrorist operation" to suppress the revolt. A fuel depot on Voronezh city was on fire, he said.

The FSB security service accused Prigozhin of attempting to launch a "civil conflict" and urged Wagner fighters to detain him.

The rapidly escalating events mark the most serious challenge yet to the Kremlin chief's long rule, and Russia's most serious security crisis since the strongman came to power in late 1999.

Putin: 'A stab in the back'

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.   

"Any internal turmoil is a deadly threat to our statehood and to us as a nation. This is a blow to Russia and to our people," Putin said. 

"This battle, when the fate of our people is being decided, requires the unification of all forces."

"Extravagant ambitions and personal interests led to treason," Putin said, referring to Prigozhin, who built his powerbase as a catering contractor to the Kremlin and now runs a private military force.    

"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.

The Russian president's speech came shortly after Prigozhin posted a message of his own, apparently filmed in Rostov-on-Don, accusing the Russian military leadership of betrayal and failure in the ongoing battle against Ukrainian forces.

Prigozhin said he and his troops were “ready to die” and blamed failures of Russia's military leadership in Ukraine for the mutiny. 

"A huge amount of territory is lost. Soldiers have been killed three, four times more than it says in documents shown to the top (leadership)."

"We are dying for the Russian people," he said. "All of us are ready to die. All 25,000, and then another 25,000," he said in the earlier audio message. 

"We will destroy everything that stands in our way," Prigozhin said, claiming that his forces had shot down a Russian military helicopter.

He called on Russians not to believe what they are being told on state television. 

Authorities in Rostov urged residents to stay home.

"Law enforcement agencies are doing everything necessary to ensure the safety of residents," Rostov governor Vasily Golubev said.

'Civil conflict' 

After Putin's speech accusing him of treason, Prigozhin launched a second broadside.

"On treason of the motherland: the president is deeply wrong. We are patriots of our motherland," Prigozhin said. "Nobody plans to turn themselves in at the request of the president, the FSB or anyone else."

Russia's headquarters in Rostov-on-Don is a key logistical base for its offensive in Ukraine.

Watching events unfold in Russia as his own forces conduct a slow-moving counteroffensive, Ukraine's President Volodymyr Zelensky said the revolt showed Russia's was failing.

"Russia's weakness is obvious. Full-scale weakness," he said.

"And the longer Russia keeps its troops and mercenaries on our land, the more chaos, pain, and problems it will have for itself later. It is also obvious, that Ukraine is able to protect Europe from the spread of Russian evil and chaos."

News photographs showed armed men surrounding administrative buildings in Rostov and tanks deployed in the city centre. It was not clear who the armed men were.

The mayor of Moscow announced that "anti-terrorist" measures were being taken in the capital and authorities said security had been tightened in several regions. 

Critical facilities were "under reinforced protection", TASS reported, citing a law enforcement source.

Missile strikes

While Prigozhin's outfit fought at the forefront of Russia's offensive in Ukraine, in recent months it has engaged in a bitter feud with Moscow's military leadership.

He has repeatedly blamed Defence Minister Sergei Shoigu and Valery Gerasimov, chief of the general staff, for his fighters' deaths.

The Russian defence ministry warned that Ukrainian troops were taking advantage of the infighting to ready an assault near the eastern hotspot of Bakhmut.

A prominent Russian general urged Prigozhin to call off efforts to remove the defence ministry leadership.

"I urge you to stop," Sergei Surovikin, commander of Russia's aerospace forces, said in a highly unusual video address.

Anti-Kremlin figure Mikhail Khodorkovsky, however, urged Russians to support Prigozhin, saying it was acceptable to back "even the devil" in taking on the Kremlin.

World leaders monitor situation

Washington-based think tank the Institute for the Study of War said the Wagner chief's attempt to force a leadership change in the defence ministry "is unlikely to succeed" given that Surovikin had denounced his call for rebellion.

However, it said Wagner's apparent capture of Rostov-on-Don "would have significant impacts on Russia's war effort in Ukraine".

US President Joe Biden was briefed on the situation in Russia and Washington "will be consulting with allies and partners on these developments", National Security Council spokesman Adam Hodge said.

France and Germany said they were closely following events. Italy warned its citizens in Russia to take care and UK Prime Minister Rishi Sunak urged "all parties" to spare civilian life.

The Kremlin said Putin has spoken to leaders in Belarus, Kazakhstan and Turkey and received reassurances of their backing. 

Questioning military operation

After years of operating in the shadows, Prigozhin has now admitted to running the elusive mercenary group Wagner and even interfering in US elections.

His forces, bolstered by tens of thousands of prison recruits, played a central role in Russia's capture of the town of Bakhmut in the eastern region of Donetsk, the longest and bloodiest battle of the conflict.

However, this week he accused Moscow's top brass of deceiving Russians about the offensive in Ukraine.

"Why did the special military operation begin?" he said. "The war was needed for the self-promotion of a bunch of bastards."

Prigozhin rose from a modest background to become part of the inner circle around Putin.

He spent nine years in prison in the final period of the USSR after being convicted of fraud and theft. In the chaos of the 1990s, he began a moderately successful business selling hot dogs.

From there he fell into the restaurant business and opened a luxury location in Saint Petersburg whose customers included Putin, then making the transition from working in the KGB to local politics.

In recent months, Prigozhin has become embroiled in a bitter power struggle with the defence ministry, accusing the military of attempting to "steal" his victories in Ukraine.