The governor of the Lipetsk region, which is about 300 miles south of Moscow, said a column of Wagner troops has been spotted in the region, Russian state media reports.

A NATO representative said the alliance was watching what was happening in Russia on Saturday.

"We are monitoring the situation," spokesperson Oana Lungescu said.

-ABC News' Rashid Haddou

A governor of the Voronezh region, about 300 miles south of Moscow, says Russia's armed forces are conducting "operational combat operations" there as part of "counter terrorism operation."

Earlier the region's government reported a column of Wagner Group fighters was moving through the region, an area between Rostov-on-Don and Moscow.

"In the bounds of the counterterrorist operation on the territory of the Voronezh region, the armed forces of the Russian Federation are conducting necessary operational combat operations," the official said. "We will inform further about the development of the situation."

-ABC News' Patrick Reevell

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.

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