The sound of ambulance and police sirens filled the air on Saturday in Rostov-on-Don -- a hub for Russia's Ukraine campaign -- where the rebellious Wagner mercenary force said it had taken over key facilities.

At a major intersection in the city centre, an armoured car with a machine gun and around a dozen men in military fatigues with silver armbands could be seen.

Armoured personnel carriers and tanks were positioned in other parts of the centre, including outside a toy shop and a circus, journalists in the city reported.

Passers-by stopped to look at the military vehicles, including transport trucks, and more armed men with silver armbands carrying rifles in resting positions.

Wagner chief Yevgeny Prigozhin has said his troops had taken control of Russia’s main military command centre for Ukraine operations as well as an airbase in the city, vowing to topple Moscow's top military leaders.

Prigozhin said his fighters captured the army HQ "without firing a single shot" and claimed to have the support of locals. 

"Why does the country support us? Because we went on a march of justice," Prigozhin said in his latest Telegram audio message. "We got to Rostov. Without a single shot we captured the building of the (army) HQ."

He said he commanded around 25,000 fighters.

Prigozhin has said his troops will be heading for the capital city, Moscow. The UK Department of Defence confirmed reports that the mercenary group convoy was en route to the capital. 

North of Rostov, 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.

Independent Russian media reported that a bridge in the south of Moscow region was blocked using sand. There were also unverified reports of a riot breaking out in one of Moscow's main prisons.

The Wagner mercenary group has relied heavily on convicts as troops, with tens of thousands of inmates added to its ranks by offering pardons in exchange for military service.  

Russian President Vladimir Putin has vowed to punish rebels and described the uprising as "high treason". 

Prigozhin pushed back.

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

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

Understanding Rostov

A port city located just over 100 kilometres from the Ukrainian border, Rostov-on-Don was founded in the 18th century and is the headquarters of Russia's southern military command.

Unlike other large Russian cities further from the front lines, Rostov-on-Don -- with a population of just over one million people -- has experienced the ripple effects of Russia's large-scale military operation in Ukraine.

One person was killed in March at an FSB building fire in Rostov-on-Don and the Rostov region has also been targeted over recent months in several drone attacks.

Regional governor Vasily Golubev called on residents not to leave their homes unless absolutely necessary and said any planned mass gatherings in the city had been cancelled.

He also asked the public not to use the M4 motorway -- the main road connection between Moscow and southern Russia -- as sections have been closed and large queues of cars have formed along it.

Golubev said public transport in the city was still functioning but routes had been changed to avoid the city centre.

"The situation that has arisen requires a maximum concentration of efforts to keep order," he said on social media.

"Law enforcement agencies are doing everything necessary to ensure the security of our region's residents."

People in Lipetsk region urged to stay indoors

Meanwhile, authorities in the southwestern Lipetsk region urged residents to stay at home on Saturday. 

"To ensure law and order and the safety of the citizens of the Lipetsk region, the operational headquarters of the region asks residents without urgent need not to leave their homes and refrain from any travel by personal or public transport," the press service of the regional government said in a statement on social media.

The announcement came as the governor of the neighbouring region of Voronezh, where the army said it was leading "combat" operations, voiced support for Putin after officials said an oil depot was on fire there.

Putin speaks to allies

A spokesperson for Putin said that the president was working in the Kremlin as normal - even as analysts monitoring his private jet's movements claimed that it was en route to St Petersburg. 

Earlier on Saturday, Putin spoke to his Belarus ally, President Alexander Lukashenko, to inform him about the situation. 

Lukashenko, who allowed Russian troops to use Belarusian territory as a launchpad for their Ukraine offensive, has remained Putin's closest ally. 

The Kremlin later said Putin also spoke to the president of Kazakhstan, Kassym-Jomart Tokayev, and the president of Uzbekistan, Shavkat Mirziyoyev.  

"The president informed them about the situation (in Russia)," Putin's spokesman Dmitry Peskov was quoted as saying by Russian news agencies. 

According to Kazakh media, Tokayev told Putin that events in Russia were an "internal affair," and Putin thanked him for his "understanding" of the situation. 

Putin also has the backing of Turkish President Recep Tayyip Erdogan, the Kremlin said. 

Putin, who has few allies on the international stage after launching the Ukraine offensive last year, called the Wagner mutiny a "stab in the back."