"Green" (Russian) on "Blue" (Allies)?
One potential aspect of increasing overt Russian support for the Assad regime, ostensibly to fight only ISIS, is the chance of "green" on "blue". The "green" being Russian combat forces, especially in the air, encountering Allied "blue" coalition aircraft and drones.
The UK Conservative government indicated before the recent announcement of a UK drone strike on British ISIS members plotting attacks, that it was a nonsense that the RAF could attack ISIS targets in Iraq and not Syria.