LONDON (Reuters) -Britain's defence ministry said on Saturday that the Russian state was facing its greatest security challenge of recent times, following what it said appeared to be a move by Wagner Group mercenary forces towards Moscow.

Russian President Vladimir Putin said in an emergency televised address on Saturday that an "armed mutiny" by the Wagner Group was treason, and that anyone who had taken up arms against the Russian military would be punished.

"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how this crisis plays out," Britain's defence ministry said in a regular intelligence update.

"This represents the most significant challenge to the Russian state in recent times," it added.

Britain said Wagner Group forces had crossed from Russian-occupied parts of Ukraine to Russia in at least two locations, and had "almost certainly" occupied key security sites in Rostov-on-Don, including the headquarters which runs Russian military operations in Ukraine.

"Further Wagner units are moving north through Voronezh Oblast, almost certainly aiming to get to Moscow," Britain said.

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

(Reporting by David Milliken; Editing by Toby Chopra)