The Wagner Group's Yevgeny Prigozhin responded to Russian President Vladimir Putin's calling him a "traitor," by saying he will not surrender or turn back.

"Putin was deeply mistaken about the betrayal. We are patriots of our homeland, we fought and are fighting," Prighozin said in an audio message. "No one is going to turn around at the request of the president, the FSB or anyone else, because we do not want the country to live longer in corruption, deception and bureaucracy."

Prigozhin accused Russia's military of targeting a Wagner column with helicopters and jets.

-ABC News' Patrick Reevell

The powerful head of Chechnya, the semi-independent Russian region, Ramzan Kadyrov, said on Saturday he supported President Vladimir Putin.

Kadyrov saiud he fully backs Putin and called Wagner Group leader Yevgeny Prigozhin's actions "treacherous."

Kadyrov has tens of thousands of heavily armed fighters. He said his forces are already moving to "zones of tension."

It raises prospect of Chechen forces fighting with Wagner Group troops.

Kadyrov has previously been friendly with Prigozhin -- his coming out in support of Putin is a boost for Putin, but also raises prospect of serious clashes in Russia.

-ABC News' Patrick Reevell

Russian leaders are "now choosing which side they are on," an advisor to Ukrainian President Volodymyr Zelenskyy said Saturday.

"The next 48 hours will define the new status of Russia," Mykhailo Podolyak said on Twitter. "Either a full-fledged Civil War, or a negotiated Transit of Power, or a temporary respite before the next phase of the downfall of the Putin regime."

He added, "A deafening 'elite' silence is in Russia so far..."

Members of the mercenary Wagner Group have begun moving north "almost certainly aiming to get to Moscow," in what amounts to the "most significant challenge to the Russian state in recent times," the U.K. Ministry of Defense said on Saturday.

"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how the crisis plays out," the ministry said on Twitter.

In the wake of Saturday's short-lived attempted rebellion against the Kremlin by the Wagner private military company, Mikhail Khodorkovsky, an exiled oligarch considered one of Putin’s best-known opponents, told ABC News he supports the mutiny and encourages Russians to back the leader of the mercenary group.

Once Russia’s richest man, Khodorkovsky, a Putin opposition activist, spent 10 years imprisoned after he challenged Putin, his case now considered a foundational moment for Putin’s regime.

When Wagner founder Yevgeny Prigozhin and his fighters marched on Moscow Saturday before making a sudden about-face, Khodorkovsky was notable among Russia’s pro-democracy opposition in calling on people to support Prigozhin, arguing that allowing him to remove Putin would create an opportunity for the democrats.

Khodorkovsky told ABC News he believed Prigozhin’s actions were a real coup attempt and that it had “seriously undermined” Putin’s power. He predicted that similar opportunities to collapse the regime will be launched soon.

"The blow to Putin’s reputation, to the authorities’ reputation, was absolutely fantastical," Khodorkovsky said. "Putin’s government today is, without a doubt, strongly undermined by what happened -- his authority, his ability to control the security services is seriously undermined."

Khodorkovsky said Prigozhin’s march on Moscow had undermined Putin’s popularity, showing neither ordinary Russians nor the security services were prepared to act to protect him.

“Along the entire route of Wagner's columns, no one in any way tried to hinder him (Prigozhin). Even the security forces did not try to stop him," Khodorkovsky said. “It showed that, in fact, inside the country, Putin has an absolute void."

Khodorkovsky said he did not support Prigozhin himself -- considering him a "war criminal" -- but that the democratic opposition should have sought to help him overthrow Putin, and then taken power from him after.

Khodorkovsky criticized other parts of the anti-Kremlin opposition who attacked him for calling on people to assist Prigozhin, saying he believed the opposition had “slept through” the opportunity and suggesting it should have sought to stage a rebellion in Moscow at the same time.

"There will definitely now be more such opportunities because of Putin’s weakening. But the next time we need to simply be more ready," said Khodorkovsky, who is living in exile in England. "If an uprising had started in Moscow to meet Prigozhin then a situation could have developed quite differently.

ABC News' Patrick Reevell