According to preliminary information shared by the Russian Foreign Ministry last week, at least 5 Russian volunteers were killed in US-led coalition's airstrike on pro-Damascus forces in Syria.
The US had said the coalition acted in self-defence when an enemy unit of 300 to 500 fighters launched an attack on an established Syrian Democratic Forces [SDF] position east of the Euphrates river in the Deir az-Zour province. "But they took direction from someone", Mattis told reporters flying back to Washington with him from a trip to Europe, according to a Pentagon transcript.
Russian officials refuse to admit Russian mercenaries are in Syria despite mounting reported evidence that they are active fighting in the interests of the Syrian government.
Russian Foreign Minister Sergey Lavrov issued a warning to Washington on Monday morning in light of an armed clash between Russian mercenaries and rebels, which received air support from the United States, earlier this month near Deir ez-Zor. Was it from external sources?
Mattis said he "understood" that Moscow had acknowledged contractors were involved, without elaborating on whether that understanding came from press reports.
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The senior diplomat's name has come up in the FBI and congressional investigations of possible collusion between the Trump campaign and Russian Federation .
At the same time, Kremlin spokesman Dmitry Peskov said earlier that he didn't rule out that Russian nationals, who do not belong to the Russian Armed Forces, could be in Syria.
The Russian government is reviewing a bill that would make private military companies legal, in order to clarify the now murky status of mercenaries engaged in conflicts overseas. There are no Russian military servicemen in that area, Russia's Defense Ministry said.
The United States and Russian Federation, while backing opposite sides in the Syria conflict, have taken pains to make sure that their forces do not accidentally collide.
"So, whatever happened, we'll try to figure it out".