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.

Russia on Friday vehemently denied having any involvement in the mysterious plane crash that is believed to have killed Wagner Group leader Yevgeny Prigozhin.

"There has been a lot of speculation around this crash [and] the tragic deaths of the plane's passengers, among them Yevgeny Prigozhin. Of course, the West presents all this speculation from a particular angle. All of that is sheer lies," Kremlin spokesperson Dmitry Peskov told reporters while answering questions during a press briefing. "One should rely on facts. For now, there are not many facts, they have yet to be established in the ongoing investigative procedures."

Prigozhin, 62, was a former close ally of Russian President Vladimir Putin. His private paramilitary organization played a key role in Putin's invasion of neighboring Ukraine before briefly launching an insurrection against the Russian military in June. Forces loyal to Prigozhin marched toward Moscow before turning back after several days.

A private jet was en route from Moscow to St. Petersburg on Wednesday when it went down near the village of Kuzhenkino in Russia's Tver region, north of Moscow. All 10 people on board were killed. Prigozhin and Wagner Group co-founder Dmitry Utkin were among the passengers identified on a flight list, according to Russian officials.

The U.S. Department of Defense said Thursday that Prigozhin was "likely" killed in the plane crash.