US Defense Secretary James Mattis today cast strong doubts on Russian influence over Bashar al-Assad’s forces in Syria following a bloody clash with US-backed forces.

The retired Marine general’s comments came one day after US special forces helped repel some 300 pro-Assad troops who reportedly targeted a Syrian Democratic Forces (SDF) facility in Deir ez-Zor with tanks and artillery. The response left about 100 attackers dead, according to the Pentagon.

Mattis said the United States had relayed their concerns about Syrian troop movements to Russian forces via a de-confliction hotline based at a US base in Qatar. Russia’s inability to stop the attack showed a lack of influence over Assad’s troops, he added, rather than a failure of the de-confliction channel.

“There were elements in this very complex battlespace that the Russians do not have control of,” Mattis said in response to a question from Al-Monitor about whether Russia had any influence over the forces that attacked the SDF facility. “You can’t ask Russia to de-conflict something they don’t control.”