In an astonishing escalation of infighting, Russia accused Wagner mercenary chief Yevgeny Prigozhin of calling for an armed mutiny, reported ABC News.

Prigozhin said on Saturday that his men had crossed the border from Ukraine into Russia and were ready to go “all the way” against the Russian military. As a long-running standoff between Prigozhin and the military top brass appeared to come to a head, Russia’s FSB security service opened a criminal case against him, TASS news agency said. It called on the Wagner private military company forces to ignore his orders and arrest him.

The move came not long after the Kremlin accused Prigozhin of calling for armed mutiny.

Prigozhin, whose frequent tirades on social media belie his limited role in the war as head of the Wagner private militia, has for months been openly accusing Defence Minister Sergei Shoigu and Russia’s top general, Valery Gerasimov, of rank incompetence and of denying his forces ammunition and support, reported CBC News.

Prigozhin urged Russians to join his forces and punish Moscow’s military leadership in the most audacious challenge to President Vladimir Putin since the start of the offensive in Ukraine last year.

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.”

The Kremlin said Putin had been informed of Prigozhin’s claims and “necessary measures are being taken”.

Prigozhin said earlier that Wagner field camps had been struck by rockets, helicopter gunships and artillery fire on orders from the chief of the military’s General Staff, General Valery Gerasimov.

He charged that General Gerasimov issued the order after a meeting with Defence Minister Sergei Shoigu, at which they decided to destroy Wagner, reported ABC News.

Early on Saturday local time, Prigozhin said in a Telegram post his men had crossed the border from Ukraine into southern Russia and were ready to go “all the way” against the Russian military. He said he and his men would destroy anyone who stood in their way.

“But we will destroy anyone who stands in our way,” he said. “We are moving forward and will go until the end.”

As the Wagner troops were said to be driving into the Russian city of Rostov, Prigozhin said that border guards greeted his mercenaries, reported ABC News.

He said young conscripts at checkpoints stood back and offered no resistance, adding that his forces “aren’t fighting against children.”

Prigozhin escalated his direct challenge to the Kremlin, calling on Friday (local time) for an armed rebellion aimed at ousting Russia’s defence minister.

He alleged, without providing evidence, that the Russian military leadership had killed many of his fighters in an air strike and vowed to punish them.

“This is not a military coup, but a march of justice,” Prigozhin declared.

The Russian security services reacted immediately by opening a criminal investigation into Prigozhin, reported ABC News.

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.

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, the state news agency Tass reported.