Members of the mercenary Wagner Group have begun moving north "almost certainly aiming to get to Moscow," in what amounts to the "most significant challenge to the Russian state in recent times," the U.K. Ministry of Defense said on Saturday.

"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how the crisis plays out," the ministry said on Twitter.

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

The U.S. announced additional sanctions targeting the Wagner Group, specifically going after gold companies and a Russian man it says are illicitly funding the Wagner Group’s operations in Ukraine and Africa.

Central African Republic based companies Midas Resources SARLU and Diamville SAU, Dubai based company Industrial Resources General Trading and Russia based company Limited Liability Company, re connected to Wagner’s leader Yevgeny Prigozhin, who is already subject to U.S. sanctions along with numerous other entities linked to the Wagner Group, according to the Department of the Treasury’s Office of Foreign Assets Control.

Andrey Nikolayevich Ivanov, a Russian national, and an executive in the Wagner Group, allegedly worked closely with Prigozhin’s entity Africa Politology and senior Malian government officials on weapons deals, mining concerns, and other Wagner Group activities in Mali, OFAC said.

"The Wagner Group funds its brutal operations in part by exploiting natural resources in countries like the Central African Republic and Mali," Brian Nelson, the U.S. under secretary of the treasury for terrorism and financial intelligence, said in a written statement. "The United States will continue to target the Wagner Group’s revenue streams to degrade its expansion and violence in Africa, Ukraine, and anywhere else."

-ABC News' Ben Gittleson