The head of the Wagner mercenary group vowed Saturday to "go to the end" to topple the Russian military leadership, whom he accused of launching strikes on his men, while the country's prosecutor general said he was under investigation for "armed rebellion".
"We are going onwards and we will go to the end," Yevgeny Prigozhin, 62, said in an audio message.
"We will destroy everything that stands in our way," he added in the most audacious challenge to President Vladimir Putin since the start of the offensive in Ukraine last year.
He later claimed his forces had shot down a Russian military helicopter.
"A helicopter has just now opened fire at a civilian column. It has been shot down by units of PMC Wagner," he said.
Prigozhin earlier said his forces, who have spearheaded much of Russia's offensive, had entered the southern Russian region of Rostov but did not provide any proof and AFP could not independently verify his claims.
In Moscow, authorities have tightened security measures, with critical facilities "put under reinforced protection", the TASS state-run news agency reported, citing a law enforcement source.
The FSB security service urged Wagner fighters to "take measures to detain" Prigozhin.
Putin was being given regular updates on the unfolding tensions between the Wagner group and the defence ministry, the Kremlin said.
Prosecutor General Igor Krasnov had informed Putin of "the initiation of a criminal case in connection with an attempt to organise an armed rebellion," Kremlin spokesman Dmitry Peskov said.
Missile strikes
The extraordinary developments came after Prigozhin accused Moscow of targeting his forces with deadly missile strikes.
"They (Russia's military) conducted missile strikes at our rear camps. A huge number of our fighters, our comrades died," Prigozhin said in a series of furious audio messages released by his spokespeople.
"The council of commanders of PMC Wagner has made a decision -- the evil that the military leadership of the country brings must be stopped."
He warned Russians against resisting his forces and called on them to join him, adding "there are 25,000 of us".
"We need to put an end to this mess," he said, adding, "this is not a military coup, but a march of justice".
In a statement, the FSB said: "Prigozhin's statements and actions are in fact a call to start an armed civil conflict on the territory of the Russian Federation and a stab in the back to Russian servicemen fighting pro-fascist Ukrainian forces".
While Prigozhin's outfit has spearheaded much of Russia's offensive in Ukraine, he has in recent months engaged in a bitter feud with Moscow's military leadership and has repeatedly blamed Defence Minister Sergei Shoigu and Valery Gerasimov, chief of the general staff, for his fighters' deaths.
'Provocation'
The Russian defence ministry denied Prigozhin's claims of an attack on his forces, saying the statements "do not correspond to reality", and calling them a "provocation".
It later said Ukrainian troops were taking advantage of the infighting to ready an assault near the east Ukraine hotspot of Bakhmut.
A prominent Russian general urged Prigozhin to call off efforts to remove the leadership of Moscow's defence ministry.
"I urge you to stop," Sergei Surovikin, commander of Russia's aerospace forces, said in a highly unusual video address.
"The enemy is just waiting for the internal political situation to worsen in our country. Before it is too late, it is necessary... to obey the will and order of the popularly elected President of the Russian Federation".
Kyiv said it was monitoring the infighting in Moscow.
"We are watching," the Ukrainian defence ministry tweeted, while Ukraine's military intelligence chief Kyrylo Budanov said rival Russian factions had begun to "eat each other over power and money".
Ukraine was also on high alert after new Russian missile strikes Saturday, with explosions reported in Kyiv and the eastern city of Kharkiv.
US President Joe Biden had been 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.
On Friday, Prigozhin said Moscow's forces were retreating in Ukraine's east and south following the start of Kyiv's counteroffensive early this month. That directly contradicted Putin's account that Ukraine was suffering "catastrophic" losses and that there was a lull in fighting.
"We are washing ourselves in blood," Prigozhin said.
"No one is bringing reserves. What they tell us is the deepest deception," he added, referring to the Russian military and political leadership.
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."
Rarely has such a controversial figure shot to this degree of prominence on the Russian political stage under Putin.
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.
However, in recent months, Prigozhin has become embroiled in a bitter power struggle with the defence ministry.
He has accused the Russian military of attempting to "steal" victories in Ukraine from his forces, and slammed Moscow's "monstrous bureaucracy" for slowing military gains.
