Investigators have "reasonable grounds" to believe chemical weapons have been used in Syria, according to a UN report.

The human rights group of the organisation said they had received allegations that both the government and rebels had used limited quantities of toxic substances, but most testimony related to their use by State forces.

Paul Pinheiro, the chair of the commission, said "It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator."

The inquiry examined four reported toxic attacks in March and April, interviewing victims, medical staff and refugees who had fled the affected areas.

The 29-page report on human rights abuses in the on-going civil war also described war crimes and crimes against humanity which have reached "new levels of brutality."

Leaders of Syria's military and government were accused of "a concerted policy" of violations, including the bombing of cities and executing of civilians, which investigators said they must be held accountable for.

The conflict has now raged for 26 months and claimed more than 80,000 lives.

The UN report blames both government and rebel forces for carrying out murder, torture and rape, but said that that the intensity and scale of the crimes committed by the state outweighed those committed by opposition fighters.