Long at odds with Russia’s military leaders during the war in Ukraine, Yevgeny Prigozhin, the head of the Wagner mercenary group, took his feud to a new level Friday when he leveled accusations that his fighters had been struck and vowed revenge.

The threat, made on his official Telegram channel, sparked a crisis in Russia, where the country's Federal Security Service called for members of the private military contractor to refuse Prigozhin's orders and detain him.

It also opened a criminal investigation for “organizing an armed rebellion,” the prosecutor general’s office said.

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Describing it as a stab in the back, Russian Lt. General Vladimir Alekseev told broadcaster RBK it was “a coup d’état.”

In a later television address, President Vladimir Putin labeled the revolt an “armed mutiny” and vowed to put down the effort. Calling Prigozhin’s actions “a betrayal of his country and people,” he said, “Everything that weakens Russia should be thrown aside.”

The Wagner Group was founded in 2014 by Prigozhin, 61, a close confidant of the Russian president who is known as “Putin’s chef” because his catering business routinely won contracts to cater state events.

Since then, the company has operated in Syria, Mali, the Central African Republic and other countries.

Private military companies are technically illegal in Russia, but in November Wagner opened offices in St. Petersburg emblazoned with its logo.

Prigozhin has also donned military fatigues and appeared in videos appearing to show him recruiting prisoners, or in the thick of the action on the front lines, in contrast to the city-dwelling elites he has routinely derided since the start of the war in Ukraine.

He has fostered a tough image online — appearing with a sledgehammer in one video and, according to Reuters, approving of another that showed an accused deserter being executed with a sledgehammer.

In Ukraine, his forces delivered a badly needed victory for Putin as they were crucial in the campaign to capture Bakhmut, which was left in ruins by months of conflict that saw both sides suffer huge losses.

The U.S. designated Wagner a transnational criminal organization in January, a month after National Security Council spokesman John Kirby said that it had an estimated 50,000 personnel inside Ukraine — around 10,000 contractors and 40,000 convicts from Russian prisons.

Prigozhin said in Friday's message that "we are 25,000 strong" and he invited others to join.

In Africa, the Wagner Group has operated in the civil conflict in Libya where Russia sent it military equipment including fighter aircraft and armored vehicles, so that it could get a foothold in the country, the U.S. Department of Defense said in 2020.

Wagner Group contractors were also sent to the Central African Republic, where the group was accused of executing civilians, attacking U.N. peacekeepers and targeting predominantly Muslim communities as a civil war raged.