Russian military helicopters opened fire on Saturday afternoon on a convoy of rebel mercenaries already more than half way towards Moscow in a lightning advance after seizing a southern city overnight.

President Vladimir Putin vowed to crush an armed mutiny he compared to Russia's Civil War a century ago.

Fighters from Yevgeny Prigozhin's private Wagner militia were in control of Rostov-on-Don, a city of more than a million people close to the border with Ukraine, and were rapidly advancing northwards through western Russia.

A Reuters journalist saw army helicopters open fire at an armed Wagner column that was advancing past the city of Voronezh with troop carriers and at least one tank on a flatbed truck. The city is more than half way along the 1,100-km (680-mile) highway from Rostov to Moscow.

Prigozhin, whose private army fought the bloodiest battles in Ukraine even as he feuded for months with the top brass, said he had captured the headquarters of Russia's Southern Military District in Rostov after leading his forces into Russia from Ukraine.

In Rostov, which serves as the main rear logistical hub for Russia's entire invasion force, residents milled about, filming on mobile phones, as Wagner fighters in armoured vehicles and battle tanks took up positions.

One tank was wedged between stucco buildings with posters advertising the circus. Another had "Siberia" daubed in red paint across the front, a clear statement of intent to sweep across the breadth of Russia.

In Moscow, there was an increased security presence on the streets. Red Square was blocked off by metal barriers.

"Excessive ambitions and vested interests have led to treason," Putin said in a televised address, comparing the insurrection at a time of war abroad to Russia's revolution and civil war unleashed during World War One.

"All those who deliberately stepped on the path of betrayal, who prepared an armed insurrection, who took the path of blackmail and terrorist methods, will suffer inevitable punishment, will answer both to the law and to our people."

A defiant Prigozhin swiftly replied that he and his men had no intention of turning themselves in.

"The president makes a deep mistake when he talks about treason. We are patriots of our motherland, we fought and are fighting for it," Prigozhin said in an audio message. "We don't want the country to continue to live in corruption and deceit."

In a series of hectic messages overnight, Prigozhin demanded Defence Minister Sergei Shoigu and the chief of the general staff Valery Gerasimov come to see him in Rostov.

Western capitals said they were closely following the situation in nuclear-armed Russia. The White House said President Joe Biden was briefed.

"This represents the most significant challenge to the Russian state in recent times," Britain's defence ministry said.

"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how this crisis plays out."

Putin's grip on power may depend on whether he can muster enough loyal troops to combat the mercenaries at a time when most of Russia's military is deployed at the front in southern and eastern Ukraine.

The insurrection also risks leaving Russia's invasion force in Ukraine in disarray, just as Kyiv is launching its strongest counteroffensive since the war began in February last year.

"Russia's weakness is obvious. Full-scale weakness," Ukrainian President Volodymyr Zelenskiy wrote in a social media message. "And the longer Russia keeps its troops and mercenaries on our land, the more chaos, pain and problems it will have for itself later."

PRIGOZHIN'S REVOLT

Prigozhin, a former convict and long-time ally of Putin, leads a private army that includes thousands of former prisoners recruited from Russian jails.

His men took on the fiercest fighting of the 16-month Ukraine war, including the protracted battle for the eastern city of Bakhmut.

He railed for months against the regular army's top brass, accusing generals of incompetence and of withholding ammunition from his fighters. This month, he defied orders to sign a contract placing his troops under Defence Ministry command.

He launched the apparent mutiny on Friday after alleging that the military had killed many of his fighters in an air strike. The Defence Ministry denied it.

"There are 25,000 of us and we are going to figure out why chaos is happening in the country," he said, promising to destroy any checkpoints or air forces that got in Wagner's way. He later said his men had been involved in clashes with regular soldiers and had shot down a helicopter.

Army Lieutenant-General Vladimir Alekseyev issued a video appeal asking Prigozhin to reconsider his actions.

"Only the president has the right to appoint the top leadership of the armed forces, and you are trying to encroach on his authority," he said.