According to Human Rights Watch (HRW) evidence suggests the Syrian government was behind chlorine barrel bombs hitting three Syrian towns last month.
Barrel bombs typically are large, empty oil drums filled with explosives and scrap metal and rolled out of a helicopter.
The Syrian government is the only party in the conflict with helicopters and other aircraft.
Photos on HRW’s website show explosions and, what witnessed described as “yellow smoke” which may be seen at the base of the dust cloud.
Such reports of “yellow smoke” are consistent with the release of chlorine gas from the rupture of industrial compressed gas cylinders.
According to doctors who treated the victims, the attacks killed at least 11 people. Around 500 more people had symptoms consistent with chlorine exposure.
Syria last year agreed with the US and Russia to dispose of its chemical arsenal after hundreds of civilians were killed in an August sarin gas attack near Damascus.
Just over 92 percent of Syria’s chemicals have been handed over to a joint task force.
Chlorine was not declared on Syria’s chemicals list.
If a chlorine gas attack can be proven, it could be a major weakness in Syria’s chemical agreement.