In a report, the UN outlines how 'civilian gatherings were deliberately targeted' by President Bashar al-Assad's forces.
Investigators spent six months looking into the conflict in Syria.
They found that 'in government prisons, detainees were subjected to horrific torture and sexual assault'.
The report also accuses Islamic State militants of committing war crimes.
It said I.S. was carrying out amputations, public executions and whippings in several towns and cities it had taken over.
"Bodies of those killed are placed on display for several days, terrorising the local population," the report said.
It added: "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."
International lawyer and church consultant Dr Harry Hagopian told Premier's News Hour these latest developments are upsetting but not surprising.
"There is a huge swathe called the Syrian people. Those hapless men, women and children who are caught between a rock and a hard place," he said.
He added: "They are the people getting a good beating on the head every time either side applies its bloody pernicious methods of control."