Islamic State (IS) militants have committed "mass atrocities" in Syria and have recruited children as fighters, the United Nationssays.
In a report, investigators say public executions are a "common spectacle" in areas run by IS, one of the groups fighting against Syria's government.
The report also accuses the Syrian authorities of using chemical agents in eight separate incidents this year.
The conflict between government forces and several rebel groups began in 2011.
Some 200,000 people have died since then.
'Horrific torture' The UN report details abuses by the Syrian government and several of the armed groups fighting it. The report says the Syrian air force has used barrel bombs on civilian neighbourhoods.
"In some instances, there is clear evidence that civilian gatherings were deliberately targeted" by government forces, the investigators said.
"In government prisons, detainees were subjected to horrific torture and sexual assault."
The findings are the result of interviews and evidence collected between January and July this year as part of an inquiry into human rights violations in Syria.
Among the other allegations of war crimes committed by the Syrian government of President Bashar al-Assad was the use of suspected chlorine gas, a chemical agent, in eight separate incidents in April and May of this year.
The period covered in the report coincides with the growth of IS in Syria. The group seeks to create an independent Islamic State in an area that stretches across Syria and Iraq.
It has attracted jihadists from across the region, as well as fighters from Western countries including the UK and the US.