The leaked report says helicopters dropped barrel bombs holding chlorine gas, a prohibited weapon, on the north-west province of Idlib in March 2015.
An earlier report by the Organization for the Prohibition of Chemical Weapons (OPCW) blamed the Syrian government for two other gas attacks in 2015.
The government has not yet commented.
Syria agreed to destroy its chemical weapons in 2013 under the terms of an agreement negotiated between Moscow and Washington.
