United Nations human rights investigators said on Tuesday they had "reasonable grounds" to believe that limited amounts of chemical weapons had been used by government forces in Syria.

In their latest report, based on interviews with victims, medical staff and other witnesses, they said they had received allegations that both Syrian government forces and rebels had used the banned weapons, but that at least four instances related to their use by state forces.

"There are reasonable grounds to believe that limited quantities of toxic chemicals were used. It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator," Paulo Pinheiro, who chairs the UN commission of inquiry, told a news conference in Geneva.

The commission examined four reported toxic attacks in March and April but could not determine which side was behind them.

"The witnesses that we have interviewed include victims, refugees who fled some areas, and medical staff," Pinheiro said, declining to be more specific for reasons of confidentiality.

The team, composed of more than 20 investigators, conducted 430 interviews from January 15 to May 15 among refugees in neighboring countries and by Skype with people still in Syria.

The report said that while most of the are allegations of government forces using chemical weapons in four instances, it could not rule out rebels using them.

"It is possible that anti-government armed groups may access and use chemical weapons.... though there is no compelling evidence that these groups possess such weapons or their requisite delivery systems," the report said.

"Conclusive findings - particularly in the absence of a large-scale attack - may be reached only after testing samples taken directly from victims or the site of the alleged attack," it said.

The report, covering the period from mid-January to mid-May, accused both sides of committing war crimes. On the government side, the report accused government forces and affiliated militia of committing torture, rape, forcible displacement and enforced disappearance. On the rebel side, the report accused armed groups of carrying out sentencing and execution without due process, as well as committing torture, taking hostages and pillaging.

But it said violations and abuses by the rebels "did not, however, reach the intensity and scale of those committed by government forces and affiliated militia."

"A dangerous state of fragmentation and disintegration of authority prevails in areas under anti-government armed groups control, despite attempts to fill the vacuum left by the withdrawal of the state through creating local councils," it said.

Turkey said last month that tests on Syrian war casualties arriving within its borders showed that chemical weapons had been used by Syrian forces, and that further tests were being carried out to verify the evidence.

Washington has said it views the use of chemical weapons in Syria as a "red line" but wants proof before taking any action in response. Both the rebels and Syrian President Bashar Assad's forces have accused the other of using such weapons.