The U.N. Commission of Inquiry said conclusive findings can be reached only after testing samples taken directly from victims or the site of the alleged attacks. It called on Damascus to allow a team of experts into the country.
The commission's report to the Human Rights Council on violations in Syria's conflict accused both sides of committing war crimes. In an apparent message to European countries considering arming Syrian rebels, the report warned that the transfer of arms would heighten the risk of violations, leading to more civilian deaths and injuries.
"War crimes and crimes against humanity have become a daily reality in Syria where the harrowing accounts of victims have seared themselves on our conscience," the report said. "There is a human cost to the increased availability of weapons," it added.
U.N.-Secretary General Ban Ki-moon has appointed a U.N. team to investigate alleged chemical weapons attacks in Syria after the Syrian government asked him to investigate a purported attack by rebels on March 19 on Khan al-Assal village in the northern city of Aleppo. But the Syrian government insists that a probe be limited to that incident.
Syrian soldiers were reportedly killed and injured in the incident, which the rebels blame on Syrian forces. Opposition activists have claimed more than six instances when regime forces used chemical weapons.
Ban is insisting on a broader investigation, including a December incident in Homs. He appointed Swedish chemical weapons expert Ake Sellstrom to lead a U.N. investigation. Syria has refused to allow his team into the country.
The confirmed use of chemical weapons could escalate the international response to the more than two-year-old conflict, which has killed more than 70,000 people, according to the United Nations. President Barack Obama has said their use would be a "red line," but the administration says it still looking for solid evidence.
"There are reasonable grounds to believe that chemical agents have been used as weapons," the report said. "It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator."
The report said there are allegations of government forces using chemical weapons in four instances, but also did 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.
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