On Saturday, the head of the Russian mercenary group Wagner said that his 25,000-strong force was "ready to die" as he pledged to topple the country's military leadership in Moscow.
"All of us are ready to die. All 25,000, and then another 25,000," Yevgeny Prigozhin said in a new audio message. "We are dying for the Russian people."
The Wagner Group is a private army of mercenaries battling alongside the Russian military in Ukraine.?
In recent months, Prigozhin, a one-time Putin ally, has carried out an increasingly bitter rivalry with Moscow.
Yevgeny Prigozhin vowed to "go to the end" to overturn the Russian military leadership a day after he blamed it for carrying out strikes on his men, while the country's prosecutor general said he was under probe for "armed rebellion".
"We are going onwards and we will go to the end," Yevgeny Prigozhin, 62, said in an audio message.
Prigozhin said that they "will destroy everything" that stands in their way in a daring challenge to President Vladimir Putin since the commencement of the offensive in Ukraine last year.
News agency AFP reported that he later claimed his forces had shot down a Russian military helicopter.
He earlier said his forces, who have spearheaded much of Russia's offensive, had penetrated the southern Russian region of Rostov. However, AFP could not verify his claims.
Russia has rallied military vehicles and beefed up its guard outside government installations and critical locations following the Wagner mercenary group of "armed mutiny" threats.
The TASS state-run news agency reported that critical facilities have been "put under reinforced protection" in Moscow, quoting a law enforcement source.
The Federal Security Service (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.
"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 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 said.
"Anyone who puts up resistance -- we will consider this a threat and destroy it immediately. We need to put an end to this mess. This is not a military coup, but a march of justice," he added.
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