Of those killed, 598 were fighters from the Islamic State group and another 824 from Al-Nusra Front, the Syrian affiliate of Al-Qaeda, and other rebel groups.
There were a further 710 civilians killed, of whom 161 were children and 104 women.
The Britain-based Observatory has an extensive network of informants inside Syria. It bases its tallies on Russian-caused casualties by the type of aircraft flown and the munitions used.