Wagner fighters almost certainly aim to march on Moscow, UK intel says. What happens next depends on the loyalty of Russia's security forces.

• Wagner Group fighters entered the southern Russian city of Rostov-on-Don in an apparent armed rebellion.
• Further, the UK MoD said that Wagner units are heading toward Moscow on Saturday.

Wagner Group fighters, who are in the middle of what appears to be an armed rebellion in Russia, are almost certainly aiming to march on Moscow, the UK Department of Defense said Saturday.

Scores of Wagner fighters entered southern Russia from occupied Ukraine and claimed to have taken control of crucial security sites in the key southern city of Rostov-on-Don.

Further, Wagner units appear to be moving north through Voronezh Oblast and heading towards Moscow, according to the intelligence update from the UK defense department.

"With very limited evidence of fighting between Wagner and Russian security forces, some have likely remained passive, acquiescing to Wagner," the department said.

The loyalty of Russia's security forces, particularly the Russian National Guard, will be "key to how the crisis plays out," it added.

Unverified videos have circulated on social media appearing to show Wagner fighters moving through the Voronezh region toward Moscow.

Some images have also circulated online claiming to show Russian military personnel surrendering to Wagner forces at the Bugayevka border crossing south of Voronezh.

A Russian security source told Reuters that Wagner fighters had taken control of military facilities in Voronezh, but this could not be independently verified.

Russian President Vladimir Putin called the apparent mutiny a "betrayal" in a speech on Saturday morning and said that Russia would defend itself and mutineers would be punished.

In his speech, Putin said that the situation in Rostov is "very complicated" amid reports that security has been heightened in the city and also in Moscow.

Moscow mayor Sergei Sobyanin said that anti-terrorist measures were being taken in the capital, The Guardian reported.

Russian authorities also said the M4 motorway, which connects Moscow to southern areas including Rostov-on-Don, had been closed to traffic at the border with the Voronezh region due to a military convoy moving, per the outlet.

The events are a dramatic escalation in an ongoing feud between Yevegny Prigozhin, the leader of the mercenary Wagner Group, and Russian military leadership.