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.

U.S. President Joe Biden and other G-7 leaders are set to unveil future support for Ukraine on Wednesday after the conclusion of a high-stakes NATO summit in Lithuana's capital.

"President Biden and G-7 leaders will make a major announcement alongside President Zelenskyy this afternoon outlining our long-term commitments to support the people of Ukraine," U.S. National Security Council Senior Director for Europe Amanda Sloat said during a press briefing in Vilnius on Wednesday morning. "The United States, along with G-7 leaders, will announce our intent to help Ukraine build a military that can defend itself and deter a future attack."

"The launch of this process today will start a series of bilateral negotiations with Ukraine on the reaching of bilateral security commitments to help make this a reality," she added. "In particular, this process will ensure that the military assistance we provide Ukraine to defend itself against Russian aggression continues to be part of a long-term investment in Ukraine’s future force."

That investment will include making sure "Ukraine has a sustainable fighting force capable of defending Ukraine now and deterring Russian aggression in the future, a strong and stable economy, and the help Ukraine needs to advance the reform agenda to support the good governance necessary to advance Ukraine's Euro-Atlantic aspirations, which Ukraine recommits itself to as part of this declaration," according to Sloan.

"Taken together, we believe the declaration we will announce today seeks to ensure Ukraine's future as a free, independent, democratic, and sovereign nation," she told reporters. "This multilateral declaration will send a significant signal to Russia that time is not on its side."

-ABC News' Molly Nagle