An armed rebellion led by a paramilitary organisation has been named "the most significant challenge to the Russian state" in recent years, according to UK defence officials.
Yevgeny Prigozhin, chief of the Wagner mercenary group, has vowed to take down Moscow's military leadership, with Vladimir Putin branding the actions of the private military company a "betrayal".
In its latest intelligence briefing, the Ministry of Defence (MoD) said that "over the coming hours, the loyalty of Russia's security forces... will be key to how the crisis plays out" as Wagner forces appear to be making their way towards the Russian capital.
The department tweeted that rising tensions between the Wagner group and the Russian defence ministry "escalated into outright military confrontation" in the early hours of Saturday.
"In an operation characterised by Prigozhin as a 'march for freedom', Wagner Group forces crossed from occupied Ukraine into Russia in at least two locations," the MoD said.
"In Rostov-on-Don, Wagner has almost certainly occupied key security sites, including the HQ which runs Russia's military operations in Ukraine.
"Further Wagner units are moving north through Voronezh Oblast, almost certainly aiming to get to Moscow. With very limited evidence of fighting between Wagner and Russian security forces, some have likely remained passive, acquiescing to Wagner.
"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.
"This represents the most significant challenge to the Russian state in recent times."
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