The Sunni militants comprising the Islamic State, and fighting Assad's military, were also cited for similar offenses by the independent inquiry commission established to investigate war crimes in Syria. The commission, chaired by Brazilian diplomat Paulo Pinheiro, compiled a confidential list of suspects it believes should face an International Criminal Court war crimes tribunal.
"Violence has bled over the borders of the Syrian Arab republic, with extremism fuelling the conflict's heightened brutality," said the 45-page report, released at the united Nations' Geneva, Switzerland offices.
"Forced truces, a mark of the government's strategy of siege and bombardment, are often followed by mass arrests of men of fighting age, many of whom disappear."
The report added it found "longstanding findings of systematic torture and deaths of detainees" in Syrian prisons.
The Guardian noted as many as 11,000 people have been killed in Syrian prisons from 2011 to 2013.
Pinheiro said the commission found eight examples of Syrian government use of chemical agents, most likely chlorine, against enemy troops -- the first time the United Nations has made that accusation.
The report also said the Islamic State campaign in Syria included instilling fear in the civilian population with public executions, amputations and whippings.
"In areas of Syria under (Islamic State) control, particularly in the north and north-east of the country, Fridays are regularly marked by executions, amputations and lashings in public squares," the commission said. "Executions in public spaces have become a common spectacle on Fridays in (the IS de facto headquarters) Raqqa and in Isis-controlled areas of Aleppo governorate," the report said.