Ukrainian President Volodymyr Zelenskyy said Russia appeared to be suffering "full-scale weakness" after the Wagner Group mercenaries said they'd taken hold of a key Russian city.

"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 on Twitter.

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..."

The United States is standing by the NATO communique released Tuesday and its language around Ukraine joining the alliance, despite criticism from Ukrainian President Volodymyr Zelenskyy over the lack of a timeline to do so.

"The United States clearly joined with NATO allies in agreeing to a strong positive message reaffirming that Ukraine will become a member of the alliance," U.S. National Security Council Senior Director for Europe Amanda Sloat said during a press briefing in Lithuania's capital on Wednesday morning, on the final day of a high-stakes NATO summit. "And as the communique has made clear, as the president has spoken to directly in the past, we recognize that Ukraine has already made significant progress in terms of reforms. That was part of what led to allies making the decision to say that the Membership Action Plan was no longer required for Ukraine."

"But as both the president has said and as the communique made clear, there is still the need for Ukraine to take further democratic and security sector reforms," she added.

Sloat told reporters that the U.S. has been and would continue to work with Ukraine both bilaterally and through the NATO alliance to ensure that the reforms required to join the alliance are met.

When asked to respond to Zelenskyy's criticism that the lack of a timeline was "unprecedented and absurd," Sloat defended the agreement as a significant one.

"I would agree that the communique is unprecedented, but I see that in a positive way. We joined with allies yesterday in agreeing to a very strong, positive message. We reaffirmed that Ukraine will become a member of the NATO alliance," she said, arguing that removing the Membership Action Plan requirement for Ukraine was a "very significant" step on NATO's part.

-ABC News' Molly Nagle