Russian President Vladimir Putin on Saturday vowed harsh punishment for the organizers of an armed rebellion spearheaded by mercenary chief Yevgeny Prigozhin, who led his troops out of Ukraine and into a key southern city.

Putin denounced the uprising as “a stab in the back” in an address to the nation. It was the biggest threat to his leadership in over two decades in power.

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

Wagner troops and equipment also rolled into Russia’s Lipetsk province, about 360 kilometers (225 miles) south of Moscow, where authorities “are taking all necessary measures to ensure the safety of the population,” said regional Gov. Igor Artamonov, via Telegram. He did not elaborate,

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 speech, Putin called the actions 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 said his fighters would not surrender, as “we do not want the country to live on in corruption, deceit and bureaucracy.”

“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.

Prigozhin’s private army, known as Wagner, has been fighting alongside regular Russian troops in Ukraine. His goals weren’t immediately clear, but the rebellion marks an escalation in his struggle with Russian military leaders, whom he accused of botching the war in Ukraine and hobbling his forces in the field.

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

Prigozhin posted video of himself at the military headquarters in Rostov-on-Don and claimed his forces had taken control of the airfield and other military facilities in the city. Other videos on social media showed military vehicles, including tanks, on the streets.

“We didn’t kill a single person on our way,” Prigozhin said in one of his several messages posted as the day went on, adding that his forces seized the military headquarters “without a single gunshot.” His claims could not be independently verified. The Russian authorities haven’t reported any casualties so far, either.

The rebellion comes at a time when Russia is “fighting the toughest battle for its future,” Putin said, 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 the 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.

It was not immediately clear how Prigozhin was able to enter the southern Russian city or how many troops he had with him.

Prigozhin said he wanted to punish Defense Minister Sergei Shoigu after he accused Russian government forces of attacking Wagner field camps in Ukraine with rockets, helicopter gunships and artillery. He claimed that “a huge number of our comrades got killed.”

Prigozhin said Wagner’s forces shot down a Russian military helicopter that fired on a civilian convoy, but there was no independent confirmation of that.

He alleged that Gen. Valery Gerasimov, chief of the General Staff, ordered the attacks following a meeting with Shoigu, where they decided to destroy Wagner.

The Defense Ministry denied attacking the Wagner camps.

Prigozhin said he had 25,000 troops under his command and urged the army not to offer resistance.

The 62-year-old Prigozhin, a former convict, has long ties to the Russian leader and won lucrative Kremlin catering contracts that earned him the nickname “Putin’s chef.”

He gained attention in the U.S. when he and a dozen other Russian nationals were charged with operating a covert social media campaign aimed at fomenting discord ahead of Donald Trump’s 2016 presidential election victory. He formed the Wagner mercenary group, which sent military contractors to Libya, Syria, several African countries and eventually Ukraine.

After Putin’s address, in which he called for unity, officials sought to reiterate their allegiance to the Kremlin and urged Prigozhin to back down.

Vyacheslav Volodin, speaker of the lower house of parliament, said lawmakers “stand for the consolidation of forces″ and support Putin, adding that “Wagner fighters must make the only right choice: to be with their people, on the side of the law, to protect the security and future of the Motherland, to follow the orders of the commander-in-chief.”

Foreign Ministry spokeswoman Maria Zakharova echoed that, saying in a Telegram post that “we have one commander in chief. Not two, not three. One.″

Ramzan Kadyrov, the strongman leader of the Chechnya region who used to side with Prigozhin in his criticism of the military, also expressed his full support of Putin’s “every word.”

“We have the commander in chief, elected by the people, who knows the situation to the slightest detail better than any strategist and businessman,” Kadyrov said. “The mutiny needs to be suppressed.”

While the outcome of the confrontation was still unclear, it appeared likely to further hinder Moscow’s war effort as Kyiv’s forces probed Russian defenses in the initial stages of a counteroffensive. The dispute, especially if Prigozhin were to prevail, also could have repercussions for Putin and his ability to maintain unity.

Wagner forces have played a crucial role, capturing the eastern city of Bakhmut, an area where the bloodiest and longest battles have taken place. But Prigozhin has increasingly criticized the military brass, accusing it of incompetence and of starving his troops of munitions.

Zelenskyy noted the rebellion in his Telegram channel and said “anyone who chooses the path of evil destroys himself.”

“For a long time, Russia used propaganda to mask its weakness and the stupidity of its government. And now there is so much chaos that no lie can hide it,” he said. “Russia’s weakness is obvious. Full-scale weakness. And the longer Russia keeps its troops and mercenaries on our land, the more chaos, pain and problems it will have for itself later.”

Prigozhin’s actions could have significant implications for the war. Orysia Lutsevych, the head of the Ukraine Forum at the Chatham House think tank in London, said infighting between the Defense Ministry and Wagner will create confusion and potential division among Russian forces.

“Russian troops in Ukraine may well now be operating in a vacuum, without clear military instructions, and doubts about whom to obey and follow,″ Lutsevych said. “This creates a unique and unprecedented military opportunity for the Ukrainian army.”

Military trucks and armored vehicles were seen in central Moscow early Saturday, and soldiers with assault rifles were deployed outside the main Defense Ministry building. The area around the presidential administration near Red Square was blocked, snarling traffic.

But even with the heightened military presence, downtown bars and restaurants were filled. At one club near FSB headquarters, people were dancing in the street near the entrance.

Prigozhin, whose feud with the Defense Ministry dates back years, had refused to comply with a requirement that his forces sign contracts with the ministry before July 1. He said Friday he was ready for a compromise but “they have treacherously cheated us.”

Col. Gen. Sergei Surovikin, the deputy commander of the Russian forces in Ukraine, urged Wagner troops to stop any move against the army, saying it would play into the hands of Russia’s enemies who are “waiting to see the exacerbation of our domestic political situation.”

In Washington, the Institute for the Study of War said “the violent overthrow of Putin loyalists like Shoigu and Gerasimov would cause irreparable damage to the stability of Putin’s perceived hold on power.”

Western countries monitored developments closely. U.S. Secretary of State Antony Blinken spoke with his counterparts in the other G7 countries and the European Union’s foreign affairs representative, his spokesman said, adding that Blinken “reiterated that support by the United States for Ukraine will not change.”

Latvia and Estonia, two NATO countries that border Russia, said they were increasing security at their borders.

The Kremlin said Putin spoke by phone with the leaders of Turkey, Belarus, Kazakhstan and Uzbekistan about the events.