Islamic State militants have committed "mass atrocities" in Syria, including the recruitment of children as fighters, the United Nations says.
Investigators have also accused the Syrian government of using chemical agents in eight separate incidents in western Syria this year.
Islamic State (IS), which now controls areas of Syria, is one of the groups fighting President Bashar al-Assad.
Some 200,000 have died since the conflict began in early 2011.
The findings are the result of six months of interviews and evidence collected between January and July this year as part of an inquiry into human rights violations inside Syria.
The period covered in the report coincides with the growth of the IS group in Syria, which seeks to create an independent Islamic State in an area that stretches across Syria and Iraq.
Investigators says IS is waging a campaign of fear in northern Syria, including carrying out amputations, public executions and whippings.
"Bodies of those killed are placed on display for several days, terrorising the local population," the report said.
"Women have been lashed for not abiding by IS's dress code. In Raqqa, children as young as 10 are being recruited and trained at IS camps."
Paulo Pinheiro, the chairman of the commission, warned of grave implications for the entire region.
"The international community's failure in its most elemental duties - to protect civilians, halt and prevent atrocities and create a path toward accountability - has been matched on the ground by an abandonment of even the pretence of an adherence to norms of international law," he said.
The investigators warned of a continued spread of violence form Syria's borders to neighbouring countries.
One of the investigators, Carla del Ponte, has urged world powers to refer Syria's war crimes to the International Criminal Court, calling it an important first step as it was in Rwanda and the Balkans.