The Conflict Intelligence Team (CIT), a respected group of Russian online researchers, has named 10 citizens killed in the USA air strikes.
After US-backed rebels clashed with Russian mercenaries earlier this month, Moscow is warning Washington against "playing with fire" in Syria.
Moscow on Tuesday said dozens of Russian citizens were injured after a US drone attack this month in eastern Syria, amid mounting reports of casualties among Russian nationals in the country.
"It is not the place of the foreign ministry to assess the legality and legitimacy of their decisions", it added.
Mercenaries not directly affiliated with the Russian military may be convenient for Moscow's interests in Syria while assuring deniability of government involvement. "We'll work with obviously anyone who can answer that question".
"But they took directions from someone".
Still, Russian officials deny they deploy private military contractors in Syria, saying Moscow's only military presence is a campaign of air strikes, a naval base, military instructors training Syrian forces, and limited numbers of special forces troops.
"Was it local direction?" Russian officials have told reporters that five Russian citizens may have been killed in clashes with US -led coalition forces. Don't ask me, I don't know.
But according to people familiar with the deployment, Russian Federation is using large numbers of the contractors in Syria because that allows Moscow to put more boots on the ground without risking regular soldiers whose deaths have to be accounted for.