Russian President Vladimir Putin was briefed Saturday on an "attempted armed rebellion," the Kremlin said, after the leader of the mercenary Wagner Group claimed control of military facilities in Rostov-on-Don, a key Russian city near the Ukrainian border.

Forces loyal to Yevgeny Prigozhin, the mercenary leader, were traveling north toward Moscow in the "most significant challenge to the Russian state in recent times," the U.K. Ministry of Defense said on Saturday.