Two men who appeared in a video posted by Daesh last month are believed to have been members of a shadowy mercenary force known as "Wagner" which the Russian army has been using to assist in its operations in Syria.
The men named Roman Vasilievich Zabolotny and Grigory Mikhailovich Surkanov were captured near the eastern city of Deir Ez-Zor. However, the Russian Defence Ministry denied that any of its soldiers were missing. Yet, the men's relatives and friends told the Russian media that they did not go to Syria with the Russian army, but as members of the Wagner group.
The group is believed to have played an essential role in Russian operations in Syria.
Although Russian law prohibits the employment of private military companies, in late 2015, independent news website in St. Petersburg, Fontanka.ru reported that former Special Forces officer Dmitry Utkin had been recruiting former soldiers to participate in the Wagner group.
Mark Galeotti, an expert on Russian security at the Institute of International Relations in Prague, said the group, which includes nearly 2,500 men, is believed to have played a prominent role in the operations that took place around Palmyra fighting alongside Syrian forces.
And while the Russian army does not recognise the Wagner team, Utkin appeared last year in a photograph with President Vladimir Putin at a Kremlin reception for military officers in honour of Defender of the Fatherland Day.
This year, the US added Utkin's name to a list of officials sanctioned for involvement in the Ukraine conflict of 2014, where the group is said to have got its start.
"They serve to solve a concrete problem: have no casualties," says Alexander Golts, a military analyst told the Economist.
On the ground, the Russian armed forces admitted the killing of 41 soldiers, including a general who was killed in September in a bombing near Deir Ez-Zor.
Deputy Director of the Institute for Political and Military Analysis, Alexander Khramchikhin, said on the ground, the force functions as a "pseudo-private" military company, receiving direction from the Russian army.