Chlorine was probably used as a weapon in the Syrian town of Saraqeb in early February, according to the international chemical weapons watchdog.
The Organisation for the Prohibition of Chemical Weapons released details in its latest report on poison gas being unleashed in Syria's civil war.
The OPCW is not mandated to apportion blame for the attack.
The organisation said its fact-finding mission probing alleged use of chemical weapons in Syria "determined that chlorine was released from cylinders by mechanical impact in the Al Talil neighbourhood of Saraqib".
The mission is also investigating allegations that poison gas was used in Douma, near the capital Damascus, in a deadly April 7 attack. It has not yet issued a report on that attack.