In early September, Russia threatened operations or precision strikes within the deconfliction zone, home to deployed US soldiers, after accusing the US-led coalition there of harboring terrorists. Responding to these threats, the US carried out a large show-of-force exercise. Since these exercises, there has been no further threats or contact regarding the zone that carries significant strategic potential as it would connect Damascus with Tehran-controlled territory. 

Source: Russia backs off threats after US Marine Corps show of force in Syria – Business Insider Deutschland