Barrel bombs filled with chemicals were dropped in Syria eight times in April, according to a UN report which points the finger of blame at government helicopters.
UN investigator Carla Del Ponte, who spent months researching the war, said both Islamic State insurgents and the Syrian government were guilty of war crimes in the conflict.
She has now called on world leaders to refer the findings to the international criminal court.
The report found that chemical agents - most likely chlorine - were dropped on several occasions, with "reasonable grounds" to believe they came from Syrian government helicopters.
It also revealed Islamic State forces had carried out public executions, displayed bodies on crucifixes, and stoned women, and said analysis of 27,000 photographs apparently taken in Syrian jails supported the UN's "longstanding findings of systematic torture and deaths of detainees".
Islamic State is one of several groups fighting the government of Syrian President Bashar al-Assad.