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.

Ukrainian President Volodymyr Zelenskyy and U.S. President Joe Biden are set to meet in Lithuania’s capital on Wednesday afternoon, a day after NATO leaders announced during a summit that Ukraine will be allowed to join the alliance "when allies agree and conditions are met" but didn't offer a timeline.

Earlier Wednesday, Zelenskyy held a joint press conference with NATO Secretary-General Jens Stoltenberg, fielding many questions from reporters about Ukraine's path to NATO membership. The Ukrainian president took a noticeably softer tone compared to his remarks the previous day criticizing the lack of a timeline as "unprecedented and absurd."

Zelenskyy told reporters it's difficult as NATO partners are living under different conditions, whereas in Ukraine "survival" matters. He said he understands some people are "afraid" to talk about Ukraine joining NATO because "nobody is willing to have a world war." He acknowledged that his country cannot be a member of the alliance while a war is going on within its borders, but he said "signals are important."

When asked about his upcoming meeting with Biden and how he plans to convince the U.S. president that Ukraine is ready for NATO membership, Zelenskyy responded with gratitude to the United States and confidence that Ukraine will join the alliance once Russia's war is over.

"I'm grateful to President Biden and to the Congress and to the people of United States that are truly the leaders in support and assistance to Ukraine. We highly appreciate this," the Ukrainian president told reporters. "Not planning to find any arguments for making sure that President Biden would see us in NATO. I believe that those arguments, they should be mutual because it's all about this security, the East, the European continent, the Eastern Flank of NATO. And I believe that NATO needs us just as we need NATO. And I believe that this is absolutely fair. I am confident that after the war, Ukraine will be in NATO. We will be doing everything possible to make it happen so that we with the United States would have a same understanding and same vision."

-ABC News' Molly Nagle and Joe Simonetti