In a report, investigators also accuse 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 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.
Children Being Recruited
In their report, UN investigators said IS was waging a campaign of fear in northern Syria, including amputations, public executions and whippings.
“Bodies of those killed are placed on display for several days, terrorising the local population,” the document says.
“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.”
On Wednesday IS’s supporters tweeted pictures allegedly showing militants executing Syrian army soldiers after capturing the government Tabqa airbase near Raqqa in eastern Syria. The pictures have not been verified.
Among the 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 report also detailed the use of barrel bombs by the Syrian Air Force, which were dropped on civilian neighbourhoods
The chairman of the UN panel, Paulo Pinheiro, said the international community had failed “in its most elemental duties – to protect civilians, halt and prevent atrocities and create a path toward accountability”.
One of the investigators, Carla del Ponte – a former chief prosecutor of two UN war crimes tribunals – has urged world powers to refer Syria to the International Criminal Court.