Between 7 and 10 february an estimated 500 pro govt Syrian militiamen supported by tanks and artillery attacked a SDF base near the town of Kasham. The base had a number of US Special Forces present which resulted in a reaction of F-15 and AC-130 gunships. An estimated 100 of the militiamen were killed and one SDF trooper was injured. On the 8th of february US Defense officials claimed Russian PMC's were involved in the clash which was later confirmed by Syrian media and Russian social media. Between 5-10 of them have been killed to which the Russian government has not replied at all.
http://www.dailystar.com.lb/News/Middle-East/2018/Feb-12/437779-russians-dead-in-battle-in-syrias-east.ashxhttps://www.almasdarnews.com/article/us-attack-pro-govt-forces-deir-ezzor-killed-10-russians-photos/
