The United States pledged on Saturday to investigate a February 7 attack involving Russian citizens against American-allied forces in Syria. US Defense Secretary Jim Mattis said: “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 added that Russian officers the US coordinated with were "apparently" unaware of the attack. "But they took directions from someone. Was it local directions? Was it from external sources? Dont ask me, I dont know. But I doubt that 250 to 300 people, all just excited on their individual self, suddenly crossed the river in enemy territory, started shelling a location and maneuvering tanks against it. "So, whatever happened, well try to figure it out. Well work with obviously anyone who can answer that question." Russia has acknowledged that five of its citizens were killed in a US-led coalition bombing in the eastern Deir al-Zour province after they were part of a group of pro-regime combatants which led an attack on positions held by the Syrian Democratic Forces. At the time of the attack on February 7, the Russian defense ministry insisted it had no servicemen in the eastern province of Syria. The US has estimated about 100 pro-Syrian regime forces were killed by US strikes to repel the attack. Many Russian citizens are fighting in Syria as mercenaries working for a private military company called Wagner according to numerous reports. Reuters has reported that about 300 men working for a Kremlin-linked Russian private military firm were either killed or injured in Syria. Mercenaries not directly affiliated with the Russian military may be convenient for Moscows interests in Syria while assuring deniability of government involvement. 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. But according to people familiar with the deployment, Russia 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. The contractors, mostly ex-military, carry out missions assigned to them by the Russian military, the people familiar with the deployment said. Most are Russian citizens, though some have Ukrainian and Serbian passports.