In their latest report, they said they had received allegations that Syrian government forces and rebels had used the banned weapons, but that most testimony related to their use by state forces.
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The commission examined four reported toxic attacks in March and April but could not determine which side was behind them.
"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.
Man injured from alleged chemical attack
"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.
President Bashar Assad's government and its opponents have accused each other of using chemical weapons.
The UN team 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.
Vitit Muntarbhorn, one of its members, said the team had cross-checked testimony about chemical weapons and viewed videos including on YouTube.
But findings remained inconclusive and it was vital that a stalled separate team of experts named by UN Secretary General Ban Ki-moon be given full access to Syria to collect samples from victims and sites of alleged attacks, the team said.
In any case, atrocities committed with conventional weapons far outweighed any casualties from the use of chemical agents, Pinheiro said, noting the absence of a large-scale toxic attack.
'New Levels of brutality'
"The conflict in Syria has reached new levels of brutality", the 29-page report said. "War crimes, crimes against humanity and gross human rights violations continue apace."
Syrian leaders must be held accountable for directing a policy that includes besieging and bombing cities and executing civilians, the investigators said.
Injured from alleged chemical attack by rebels in Aleppo (Photo: AP)
"The documented violations are consistent and widespread, evidence of a concerted policy implemented by the leaders of Syria's military and government," they said in their fifth report on the 26-month-old war that has killed more than 80,000.
Government forces and allied militia have committed murder, torture, rape and other inhumane acts, the report said.
For the past two weeks, Syrian government forces have laid siege to the border town of Qusair where agencies say hundreds of wounded and other civilians are trapped in dire conditions.
Syrian rebels and allied foreign militants have murdered civilians as well as captured soldiers, often after "show trials" in an increasingly sectarian conflict, the report said.
"They continue to endanger the civilian population by positioning military objectives in civilian areas," it added.
However, war crimes by rebels, including murder, torture and hostage-taking, did not reach the intensity and scale of those committed by government forces and affiliated militia.
The team called on the UN Security Council to ensure that those responsible for crimes face justice, including by possible referral of Syria to the International Criminal Court.
"Accountability will come, it will come in any case," said Carla del Ponte, a former UN war crimes prosecutor and a member of the commission.
At least 17 massacres were committed in the period under review, making a total of 30 since September, the report said.
Dozens of women and children were killed in May in the coastal villages of Baida and Banias, where evidence links the slaughter to government-backed militia, it said.
Eleven kneeling, blindfolded men were shot in the back of the head in Deir al-Zor province by al-Qaeda -linked Nusra Front rebels, the report said, citing a video that appeared in May.
Regarding a separate incident near Deir al-Zor in which the evidence also points to rebels, it said: "Video footage emerged showing a child participating in the beheading of two kidnapped men. Following investigation, it is believed that the video is authentic and the men were soldiers, killed as depicted."
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