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.

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 U.N. commission of inquiry, told a news conference in Geneva.

“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.