The Washington-Moscow agreement on reducing the possibility of accidental aerial combat between the two  has only one purpose: to look responsible to the public. How can American support to the rebels not conflict with Russia's support of Assad's government. Assad needs a rebel defeat, and Washington needs a regime change. The agreement was nothing but window dressing for a limited conflict between two super powers. Follow up the agreement with the recent visit of Assad to Moscow for reassurance that he will continue to rule Syria. America's hands are tied in this relationship.
Washington is supporting a rebel army including some elements sworn to destroy America against a legitimate government. This harkins back to the days when the CIA led overthrows of legitimate governments in Iran and Guatemala.
The best outcome for Washington is to accept the fact that Assad will survive due to Moscow's support which will continue since she wants to maintain access to her only Mediterranean naval base, Tartus.Instead, make an agreement with Russia to work together to destroy ISIS. Once done, Washington can try a new tactic to deal with a Russian controlled Syria. And there may be room to negotiate since Russia's control of Syria will bleed Moscow's coffers of more rubles than she can afford.