"I understand that the Russian government now is saying that some of their non-military forces, contractors, were involved in that still unexplained attack," Mattis said Saturday while flying back to Washington after a weeklong tour of Europe.
"But they took directions from someone. Was it local directions? Was it from external sources? Don't ask me, I don't 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,” he added.
"So, whatever happened, we'll try to figure it out. We'll 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 province of Dayr al-Zawr in eastern Syrian after they were part of a group of pro-government forces which led an attack on positions held by the Syrian Democratic Forces (SDF).
At the time of the attack on February 7, the Russian defense ministry insisted it had no troops in Dayr al-Zawr.
The US has around 2,000 military personnel in northern Syria supporting the SDF, which is an alliance of militias in northern and eastern Syria and largely dominated by the Kurdish People’s Protection Units (YPG).
The US and its allies have been bombarding what they call Daesh positions inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate.
The strikes, however, have on many occasions resulted in civilian casualties and failed to fulfill their declared aim of countering terrorism.
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.
Russia’s assistance has proved to be a major success as Syria has managed to liberate key areas over the past years, including the second largest city of Aleppo, relying on Russia’s air cover and military advice.