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."

Ukrainian President Volodymyr Zelenskyy arrived for his first NATO summit meeting in Lithuania's capital on Wednesday where he received a jovial welcome from world leaders eager to shake his hand -- a mood in keeping with the more conciliatory tone Zelenskyy has struck since landing in Vilnius.

The Ukrainian leader ultimately made a beeline for U.S. President Joe Biden, as NATO Secretary General Jens Stoltenberg created a path for him through the crowd of world leaders. The two men -- Biden in a suit and Zelenskyy in a dark green polo -- greeted each other with smiles and shook hands before chatting briefly.

Everyone eventually took their seats for the meeting of the NATO-Ukraine Council -- the first of its kind -- and Stoltenberg began his remarks with a warm welcome to Zelenskyy, whom he called "our dear friend."

"Welcome to this first meeting of the NATO-Ukraine Council," Stoltenberg said. "This is truly a historic moment, allies and Ukraine sitting side-by-side as equals to address our common vision of Euro-Atlantic security."

"And I'm really honored to welcome in our midst our dear friend, Volodymyr Zelenskyy, president of Ukraine. Welcome to Lithuania and it's great to have you here," he added.

Stoltenberg proceeded to give a very clear message about where NATO stands on Ukrainian membership, though no details on the pathway as has been a point of contention. He noted the summit has "reaffirmed that Ukraine will become a member of the alliance and we have made decisions to bring Ukraine closer to NATO."

"Today, we meet as equals," he said. "And I look forward to the day we meet as allies."

-ABC News' Cheyenne Haslett