The airstrikes killed 1,465 ISIL militants, mostly non-Syrians, and 73 members of the Nusra Front, an al-Qaeda branch operating in Syria, according to the London-based monitoring group.
The attacks also killed 62 civilians, including eight children, the group said.
The monitor added that the real number of ISIL deaths is likely to be higher than it has been able to document, as there is "an absolute secrecy on casualties" and limited access to areas that have seen clashes and bombardments.
Islamic State (ISIL) has seized vast territories across Syria and Iraq, gaining notoriety for its brutal methods, including mass executions.
The group's positions in Syria have been targeted by the US-led international coalition since September last year. Coalition airstrikes against the group are also being conducted in neighboring Iraq.
