Russia's security agency FSB said it has launched an investigation over Wagner Group's boss Yevgeny Prigozhin's comments, which "amounted to start armed civil conflict" in the country, the Interfax news agency reported Friday.
The head of the private army had called for a fight against Moscow's military leadership on Friday evening, according to the news agency.
Kremlin spokesperson Dmitry Peskov said President Vladimir Putin had been informed about the case.
In a sign of how seriously the Kremlin was taking the threat, riot police and the National Guard have been scrambled to tighten security at key facilities in Moscow, including government agencies and transport infrastructure, Tass reported.
Russia's chief prosecutor said the criminal investigation was justified and that an armed rebellion charge carries a penalty of up to 20 years imprisonment.
Ukraine army says 'we are watching,' amid infighting in Russia
The Ukrainian army said it was following infighting in Moscow after the head of the Wagner mercenary group accused Russia's military of attacking one of the private fighting group's bases.
"We are watching," the Ukrainian defense ministry tweeted, while Ukraine's military intelligence chief Kyrylo Budanov said rival Russian factions had begun to "eat each other over power and money."
Earlier, Prigozhin had accused Moscow's military leadership of attacking his mercenary units and threatened countermeasures.
Russia's Defense Minister Sergei Shoigu had Wagner camps in the hinterland attacked with artillery, helicopters and missiles, Prigozhin said in a voice message circulated by his press service on Telegram on Friday.
He said he had 25,000 men under orders who would now investigate why such arbitrariness was prevailing in the country.
"Anyone who tries to resist us, we will consider them a threat and kill them immediately," Prigozhin threatened. The Russian Defense Ministry denied an attack.
The deputy commander of Russia's Ukraine campaign, Gen. Sergei Surovikin, on Friday, urged the fighters of the Wagner private militia to give up their opposition to the military leadership and return to their bases.
"I urge you to stop," he said in a video message posted on Telegram. "The enemy is just waiting for the internal political situation to worsen in our country."
According to Prigozhin, Shoigu came specially to Rostov-on-Don, a city of over 1 million people near the Ukrainian border, to lead the operation against Wagner.
"At 9 p.m. (6 p.m. GMT) he fled – cowardly as a woman – not to explain why he had helicopters take off and missile strikes to kill our boys. This beast will be stopped," Prigozhin said.
He spoke of a "large number" of dead but did not give an exact number of the mercenaries allegedly killed in the strike.
The Defense Ministry immediately denied the allegations. All allegations were false and a "provocation," the ministry said in a statement circulated in the evening.
The National Anti-Terrorism Committee also called the allegations baseless.
"The allegations spread in the name of Yevgeny Prigozhin have no basis in fact. That is why the FSB has initiated criminal proceedings on the basis of these statements for calling for an armed coup," the agency's widely distributed statement said.
In addition to the FSB, practically all other Russian security organs belong to the committee.