The Russian Defense Ministry said in a statement earlier this month that a pro-Syrian government militia unit conducted a reconnaissance and search operation on February 7 in the vicinity of an oil refinery in the province of Deir al-Zour to identify and destroy a bandit group responsible for shelling government troop positions, and came under fire from the USA -led coalition and its local allies.
Russia has backed Syria's fight against terror since late 2015 when Russian air force and troops arrived in the Arab country at an official request of Damascus to help the government fight against Daesh and other Takfiri militants.
Asked Saturday whether Russian Federation has responsibility for contractors in Syria, Mattis said he needs more information to "understand and answer that authoritatively".
Moscow has provided no details of what the mercenaries were doing, but accounts of the operation have emerged in interviews with family members.
Russia is watching with alarm the attempts of the United States to implement its plans for sharing Syria, Russian Foreign Minister Sergei Lavrov said at the opening of a conference at the worldwide discussion club "Valday", quoted by TASS.
After US-backed rebels clashed with Russian mercenaries earlier this month, Moscow is warning Washington against "playing with fire" in Syria.
Under the deal, the company would capture and secure oil and gas infrastructure in Syria in exchange for a 25 percent share in future resource production.
Families of the dead men have demanded an explanation for what happened.
"I understand that the Russian government now is saying that some of their not-military forces, contractors, were involved in that still unexplained attack", he said.
Zarif for his part said Iran is concerned about a "new wave" of foreign intervention in Syria led by the U.S. after the defeat of Islamic State.
"Was it local direction?"
"But I doubt that 257 people all just decided on their individual own selves to suddenly cross the [Euphrates] River into enemy territory and start shelling a location and maneuvering tanks against it", he added. Don't ask me, I don't know.