Russian President Vladimir Putin said moves taken by Wagner Group leader Yevgeny Prigozhin, a longtime ally, to bring his troops into a key Russian city amounted to a "stab in the back."

Putin didn't mention Prigozhin by name, but said that "necessary orders have been given" to defend Russia in a recorded address aired on Russian television on Saturday.

"Actions that divide our unity are in essence defeatism before one's own people," he said. "This is a stab in the back of our country and our people."

-ABC News' KJ Edelman

Russian President Vladimir Putin has been briefed by the country's security agencies about what was referred to as an "attempted armed rebellion," according to Russia's state-run media.

The late-night statement from Putin's spokesman suggested that the Kremlin considered Wagner Group's move into Rostov-on-Don, a key Russian city close to the border with Ukraine, to be a "rebellion."

Wagner's founder Yevgeny Prigozhin in an audio message on Friday claimed his forces would now punish Russia's defense minister and chief of general staff, telling other units to stand down and not offer resistance.

"Special services, law enforcement agencies, namely the Ministry of Defense, the FSB, the Ministry of Internal Affairs, Rosgvardiya, in round-the-clock mode, constantly report to the president on the measures taken in the context of the implementation of the instructions previously given to him," Kremlin spokesperson Dmitry Peskov said on Saturday.

-ABC News' Tanya Stukalova and Patrick Reevell

The Wagner Group is a private military organization run by Yevgeny Prigozhin, a longtime ally of Russian leader Vladimir Putin, with tens of thousands of fighters, according to U.S. officials.

Earlier this year, the U.S. labeled the group a "significant transnational criminal organization" and levied new sanctions, while human rights observers this week said they suspected Wagner fighters were linked to the mass killing of people in Mali last year.

Government reports, statements from U.S. officials and insights from experts, as well as other sources, shed light on the Wagner group's history and goals, its alleged wrongdoings and its importance to Russia -- in Ukraine and elsewhere in the world.

-ABC News’ Nathan Luna, Leah Vredenbregt and Ivan Pereira

Yevgeny Prigozhin, leader of the Wagner Group, said on Saturday that the headquarters of the Southern Military District and all military facilities in Rostov-on-Don were under his control.

Prigozhin in a video demanded that Kremlin bring him Chief of the General Staff Valery Gerasimov and Sergei Shoigu.

He also threatened in the video that he would go to Moscow.

"We will destroy anyone who stands in our way," he said in one of a series of video and audio recordings posted on social media.

He added, "We are moving forward and will go until the end."

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.