President Vladimir Putin has called on all Wagner mercenaries and other Russian private soldiers to swear allegiance to the Russian state.
The decree, which applies to anyone participating in military activities in Ukraine, was signed two days after Wagner's leaders were killed in a plane crash.
Analysts say this latest decree by Putin is part of efforts to restore his authority after Wagner's rebellion in June.
"Putin wants to have tighter control over Wagner to make sure he doesn't face another crisis in the future," Natia Seskuria of the Royal United Services Institute told the BBC.
Described as "a step to build the spiritual and moral foundations of Russia's defense", the oath includes a promise to strictly follow the orders of commanders.
"It's a hidden message for military intelligence to find and prosecute Wagner's fighters," Petro Burkovskyi, who heads the Foundation for Democratic Initiatives, a Ukraine-based think tank, told the BBC.
It is also seen as a clear message to the fighters, as it suggests: "Either take the oath, or disarm and go to prison."
