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.

The United States "is prepared to defend every inch of NATO territory," National Security Advisor Jake Sullivan said Wednesday, amid Russia's threats of military retaliation if Sweden and Ukraine officially join the alliance.

"At the end of the day, Russia does not get a vote in who joins NATO," Sullivan told ABC News' George Stephanopoulos during an interview Wednesday on "Good Morning America."

"If Russia does choose to test Article 5, President Biden has said he is prepared to defend every inch of NATO territory," he added. "And later tonight, when he addresses a large crowd in Vilnius in a speech about his vision for Euro-Atlantic security and for American leadership, he will reinforce this point about the sanctity of Article 5."

Article 5 of the NATO treaty states that the signed parties "agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all."

Sweden and Finland, which have historically embraced neutrality, applied for NATO membership together last year despite warnings from Moscow against doing so, as Russian forces invaded Ukraine. Finland officially joined the alliance in April following Turkish ratification, but Turkey and Hungary have yet to ratify Sweden’s membership. NATO Secretary-General Jens Stoltenberg announced Monday that Turkish President Recep Tayyip Erdogan has agreed to send Sweden’s NATO accession documents to Turkey’s Parliament for approval "as soon as possible," one of the last steps in the membership process.

Meanwhile, Ukraine is pushing hard for NATO membership as Russia’s war presses on. Stoltenberg announced Tuesday that the bloc "will issue an invitation for Ukraine to join NATO when allies agree and conditions are met," pledging to speed up the process by eliminating bureaucratic hurdles, but without offering a timeline.

"All 31 allies got together yesterday to say with one voice that Ukraine's future is in NATO. The real question is just how we work down the pathway to get them in," Sullivan told ABC News on Wednesday. "So the signal that NATO sent yesterday was very positive to Ukraine about its future prospects for joining NATO, and we will work in terms of the democratic and security sector reforms that are necessary. And in the meantime, we're not just going to sit around. We're going to provide Ukraine with the weapons and military assistance it needs to defend its territory against Russia and to deter future aggression from Russia."

Ukraine has requested F-16 fighter jets and long-range missiles to aid its counteroffensive against Russian forces. Sullivan said the U.S. has already taken steps with NATO allies to begin the training of Ukrainian pilots on F-16s, which "will take some time," and then the aircraft will be transferred to Ukraine "likely from European countries that have excess F-16 supplies." As for the long-range missiles, Sullivan said the U.S. will continue to look into the issue and discuss it with Ukraine.