"The perpetrators of these violations and crimes, on all sides, act in defiance of international law," the report states. "They do not fear accountability."
The period covered by the report precedes the date of the chemical attack in Syria that has brought the possibility of military strikes in response.
Both sides in the civil war continue to fight because they believe that a military victory is possible, the report states, but it asserts such an outcome is not possible.
"There is no military solution to this conflict," it says. "Those who supply arms create but an illusion of victory."