The Wagner Group, a Russian private paramilitary organisation, has not been entirely successful in its efforts to recruit 1,500 prisoners, with many are refusing to join, a senior US defence official said on Monday.
"Our information indicates that Wagner has been suffering high losses in Ukraine, especially and unsurprisingly among young and inexperienced fighters," the official told reporters, speaking on the condition of anonymity.
The European Union has imposed sanctions on the Wagner Group, accusing it of operating surreptitiously on Moscow's behalf.
President Vladimir Putin has said the group does not represent the Russian state, but that private military contractors have the right to work anywhere in the world as long as they do not break Russian law.
The US official pointed to recent social media videos, which appeared to show Yevgeny Prigozhin, to whom the U.S. Treasury Department and the EU have said the Wagner Group is linked, trying to recruit convicts.
The video appeared to show Prigozhin attempting to recruit Russian prisoners as well as Tajiks, Belarusians and Armenians.
Reuters has not independently verified the social media videos, although BBC confirmed that it was Wagner Group leader.
Prigozhin himself is an oligarch and has previously been called "Putin's chef" as a result of dinners where he hosted the Russian President at his restaurants. He has previously denied associations with the mercenary organisation.
Wagner Group fighters have been accused by rights groups and the Ukrainian government of committing war crimes in Syria and eastern Ukraine from 2014 onward.