The report by a fact-finding mission of the Organization for the Prohibition of Chemical Weapons (OPCW), which deals with investigating the use of chemical weapons, includes eyewitness accounts of helicopters dropping barrel bombs with toxic chemicals.
Witnesses reported hearing helicopters before the attacks.
Western members of the U.N. Security Council blamed the Syrian authorities for the attacks as only they had access to helicopters.
U.S. Ambassador to the U.N. Samantha Power tweeted that "32 witnesses saw or heard sound of helicopters as bombs struck; 29 smelled chlorine. Only Syrian regime uses helos."
The investigators said 26 people heard the distinctive "whistling" sound of the falling barrel bombs containing toxic chemicals and 16 visited the impact sites and saw the bombs or their remnants. They said 29 people smelled "the distinctive odor of the gas cloud" released after the bombs hit the ground, mainly describing it "as intense, chlorine-like, similar to cleaning material used to clean toilets, but much stronger."
The 117-page report offers the most detailed findings to date regarding the use of chemical weapons in Syria, but does not say which side used them.
The report focuses on three Syrian villages, two in Idlib and one in Hama, where it says there is a "high degree of certainty" chlorine gas was used against the Syrian people.
The report includes a description of 142 videos and 189 pieces of material obtained by the investigators as well as photos of impact sites and the inner chlorine cylinder from a barrel bomb.
The findings are consistent with two previous reports by the mission.
The mission was established by the OPCW on April 29 to establish the facts surrounding allegations of the use of chlorine "for hostile purposes" in Syria. Chlorine gas is readily available and is used in industry around the world, but can also be used as a weapon.
After a closed-door meeting of the U.N. Security Council on the progress in destroying Syria's chemical weapons program, Power said the new report added credence to allegations that the Syrian government used chlorine gas as a weapon in its four-year-old civil war after pledging to give up its toxic arsenal.