Russian mercenary chief Yevgeny Prigozhin appeared to suggest he had sent an armed convoy on a 1,200-km (750-mile) charge towards Moscow on Saturday in an unlikely attempt to topple the military leadership.

Russian local officials said a military convoy was on the main motorway linking the southern part of European Russia, bordering Ukraine, with Moscow, and warned residents to avoid it.

Hours earlier, the Russian authorities had accused Prigozhin of staging an armed mutiny after he alleged, without providing evidence, that the military leadership had killed a huge number of his fighters in an air strike, and vowed to punish them.

The FSB domestic security service said it had opened a criminal case against Prigozhin for armed mutiny, a crime punishable with a jail term of up to 20 years.

The dramatic turn of events, with many details unclear, looked like the biggest domestic crisis President Vladimir Putin has faced since he ordered a full-scale invasion of Ukraine - something he called a "special military operation" - in February last year.

Prigozhin, whose Wagner militia spearheaded the capture of the Ukrainian city of Bakhmut last month, has for months been openly accusing Defence Minister Sergei Shoigu and Russia's top general, Valery Gerasimov, of rank incompetence and of denying Wagner ammunition and support in its battles in Ukraine.

As their feud appeared to come to a head, the ministry issued a statement saying Prigozhin's accusations were "not true and are an informational provocation".

Prigozhin said his actions were not a military coup. But in a frenzied series of audio messages, in which the sound of his voice sometimes varied and could not be independently verified, he appeared to suggest that 25,000 fighters were en route to oust the leaders of the defence establishment in Moscow.

Early on Friday, he had appeared to cross a new line in his increasingly vitriolic feud with the ministry, saying that Putin's stated rationale for invading Ukraine was based on lies concocted by the army's top brass.

About 2 a.m. (2300 GMT), Prigozhin posted a message on the Telegram app saying his forces had crossed into Russia from Ukraine and were in Rostov.

He said they were ready to "go all the way" against the top brass and destroy anyone who stood in their way.

Putin being briefed 'around the clock'

Around the same time, the state news agency TASS quoted Kremlin spokesman Dmitry Peskov as saying all Russia's main security services were reporting to Putin "round the clock" on the fulfilment of his orders with respect to Prigozhin.

Security was being tightened in Moscow, Mayor Sergei Sobyanin said on his Telegram channel.

In Washington, U.S. President Joe Biden has been briefed on the Russia situation, a White House spokesperson said.

About 5 a.m. (0200 GMT), the administration of the Voronezh region, on the M-4 motorway between the regional capital Rostov-on-Don and Moscow, said on Telegram that a military convoy was on the highway and urged residents to avoid using it.

Unverified footage posted on social media showed a convoy of assorted military vehicles, including at least one tank and one armoured vehicle on flatbed trucks. It was not clear where they were, or whether the covered trucks in the convoy contained fighters. Some of the vehicles were flying the Russian flag.

Footage on channels based in Rostov-on-Don showed armed men in military uniform skirting the regional police headquarters in the city on foot, as well as tanks positioned outside the headquarters of the Southern Military District. Reuters confirmed the locations shown but could not determine when the footage was shot.

In his audio messages, Prigozhin said: "Those who destroyed our lads, who destroyed the lives of many tens of thousands of Russian soldiers, will be punished. I ask that no one offer resistance...

"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.

FSB urges Wagner fighters to detain Prigozhin

The FSB said Prigozhin's statements were "calls for the start of an armed civil conflict on Russian territory and his actions are a 'stab in the back' of Russian servicemen fighting pro-fascist Ukrainian forces".

It added: "We urge the ... fighters not to make irreparable mistakes, to stop any forcible actions against the Russian people, not to carry out the criminal and traitorous orders of Prigozhin, to take measures to detain him."

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.

Army General Sergei Surovikin, the deputy commander of Russian forces in Ukraine, whom Prigozhin has praised in the past, said in a video that "the enemy is just waiting for our internal political situation to deteriorate".

"Before it is too late ... you must submit to the will and order of the people's president of the Russian Federation. Stop the columns and return them to their permanent bases," he said.

An unverified video on a Telegram channel close to Wagner showed the purported scene of an air strike against Wagner forces. It showed a forest where small fires were burning and trees appeared to have been broken by force. There appeared to be one body, but no more direct evidence of any attack.

It carried the caption: "A missile attack was launched on the camps of PMC (Private Military Company) Wagner. Many victims. According to eyewitnesses, the strike was delivered from the rear, that is, it was delivered by the military of the Russian Ministry of Defence."

Prigozhin has tried to exploit Wagner's battlefield success, achieved at enormous human cost, to publicly berate Moscow with seeming impunity, while carefully avoiding criticism of Putin.

But for the first time on Friday, he dismissed Putin's core justifications for invading Ukraine 16 months ago, something for which many Russians have been fined or jailed.

"The war was needed ... so that Shoigu could become a marshal ... so that he could get a second 'Hero' [of Russia] medal," Prigozhin said in a video clip.

"The war wasn't needed to demilitarise or denazify Ukraine," he said, referring to Putin's justifications for the war.