A senior US military official Tuesday defended an air strike on pro-Assad forces last week that reportedly killed "dozens" of Russian mercenaries. "This was self defense," Air Force Lt. Gen. Jeffrey Harrigian, commander of US Air Forces Central Command, said at a news briefing Tuesday. "This is a hostile force launching an unprovoked, coordinated attack ... against an established SDF position,” CNBC reported. Hot deconfliction line According to Harrigian, the pro-Assad forces initiated the attack by firing artillery and tanks rounds at SDF positions. He said the battalion-sized group of fighters attempted to advance on the SDF "under cover of supporting fires from artillery, tanks, and multiple launch rocket systems and mortars." Harrigian said the US-backed coalition contacted the Russians through the "deconfliction" telephone line to make them aware of the unprovoked attack. After these calls, he said the coalition officials gave the approval to conduct "strikes to destroy hostile forces." The US counterattack involved the use of F-15 fighter jets, B-52 bombers, AC-130 gunships and Apache combat helicopters as well as MQ-9 Reaper drones. Harrigian added that the deconfliction line with the Russians has been used on a daily basis in the region where the attack took place and he described conversations over the hotline as "professional." Pro-Assad forces attack not a surprise Harrigian said the attack by the pro-Assad forces was not a surprise since the US and its partners had observed them engaged in a slow buildup the week before the attack. At the same time, the American military official said the US had reminded the Russians on the deconfliction hotline about the SDF and the coalition’s presence in the area. "This was well in advance of the enemy forces attack," he said. The pro-government forces were likely seeking to seize oil fields in Khusham east of the Euphrates in Deir Ezzor province, Reuters reported, citing a US official who spoke on condition of anonymity. Dozens of Russian mercenaries killed More than 200 contract soldiers, mostly Russians fighting on behalf of Assad, died in a failed attack on a base held by US and SDF fighters in Syria’s Deir Ezzor province, two of the Russians said, according to Bloomberg. A US official told Bloomberg the death toll in the fighting was at about 100, with 200 to 300 injured, but was unable to say how many were Russians. Reuters reported the names of at least two Russian men fighting informally with pro-Assad forces who were killed in the incident in Deir Ezzor province, citing interviews with their associates. It said their associates also confirmed that "dozens" of other Russian fighters lost their lives in the same fighting on Feb. 7. There were an undisclosed number of US coalition advisors on the ground with SDF fighters during the clash, but they apparently didn’t suffer casualties. Coalition forces called in airstrikes to halt an attack from pro-Assad forces. However, Harrigian refused to speculate on the composition of the hostile force or whose control they were under but confirmed they supported the Assad regime. He also wouldn’t even confirm whether Russian nationals were among the dead in the attack. "We are focused on a singular enemy – ISIS," said the Air Force general. "We are not looking for a fight with anyone else. Meantime, the Kremlin said Tuesday that it had no information about Russian mercenaries reportedly being killed in Syria. Moscow has never openly acknowledged that private military contractors have operated in Syria alongside its Russian armed forces. Russian law officially bans mercenary service. Yet it has been widely reported that Wagner and other shadowy private military groups operating out of Russia have sent soldiers to fight abroad in several conflicts.