President Vladimir Putin vowed Saturday to defend Russia against an armed rebellion by mercenary chief Yevgeny Prigozhin, who led his troops out of Ukraine and into a key city south of Moscow.

The uprising, which Putin called “a stab in the back,” was the biggest threat to his leadership in over two decades in power.

The private army led by Prigozhin appears to control the military headquarters in Rostov-on-Don, a city 660 miles (over 1,000 kilometers) south of Moscow that runs Russian offensive operations in Ukraine, Britain’s Ministry of Defense said in an intelligence briefing.

As the fast-moving events unfolded in Russia, Ukrainian President Volodymyr Zelenskyy said Moscow is suffering “full-scale weakness” and that Kyiv was protecting Europe from “the spread of Russian evil and chaos.”

In his address, Putin called the uprising by Prigozhin, whom he did not mention by name, a “betrayal” and “treason.”

“All those who prepared the rebellion will suffer inevitable punishment,” Putin said. “The armed forces and other government agencies have received the necessary orders.”

Prigozhin called himself a patriot.

“Regarding the betrayal of the motherland, the president was deeply mistaken. We are patriots of our homeland,” he said in an audio message on his Telegram channel.

He said his fighters would not turn themselves in at the request of Putin, as “we do not want the country to live on in corruption, deceit and bureaucracy.”

Prigozhin’s private military contractor, known as Wagner, has been fighting alongside Russian troops in Ukraine. It wasn’t immediately clear what his aims were, but the rebellion marks an escalation in Prigozhin’s struggle with Russian military leaders, who he has accused of botching the war in Ukraine and hamstringing his forces in the field.

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

Prigozhin confirmed Saturday he and his troops reached Rostov-on-Don after crossing the border from Ukraine.

He posted a video of himself at the Russian military headquarters in Rostov and claimed that his forces had taken control of the air field and other military facilities in the city. Other videos posted on social media showed military vehicles, including tanks, on the streets.

Prigozhin said his forces faced no resistance from young conscripts as they crossed into Russia, saying his troops “aren’t fighting against children.”

Putin condemned the rebellion, which comes at a time when Russia is “fighting the toughest battle for its future” as western governments heap sanctions on Moscow and arm Ukraine.

“The entire military, economic and information machine of the West is waged against us,” Putin said.

Russia’s security services, including the Federal Security Service, or FSB, called for Prigozhin’s arrest after he declared an armed rebellion late Friday.

In a sign of how seriously the Kremlin took the threat, authorities declared a “counterterrorist regime” in Moscow and its surroundings, allowing restricted freedoms and enhancing security in the capital.