The UN report, based on data from four groups and the government that were cross-checked, reflects a reality of killing and torture while the civil war has "dropped off the international radar", UN High Commissioner for Human Rights Navi Pillay said.
She added: "With additional killings reported from earlier periods, in addition to the new killings that have taken place, the total is more than double the number documented a year ago. Nevertheless, it is probably an underestimate of the real total number of people killed during the first three years of this murderous conflict."
An additional 51,953 killings that were reported lacked sufficient information and were excluded from the analysis. A further significant number may not have been reported by any of the five sources.
The highest number of documented killings were recorded in Rural Damascus province (39,393), followed by Aleppo (31,932), Homs (28,186), Idlib (20,040), Daraa (18,539) and Hama (14,690).
Ms Pillay called for alleged war crimes and crimes against humanity committed by all sides in Syria's conflict to be referred to the International Criminal Court.