The global chemical weapons watchdog says it has confirmed "with utmost confidence" that mustard gas was used during an attack on a town in northern Syria in August.
The Organization for the Prohibition of Chemical Weapons said Friday the gas killed an infant and poisoned two others during the fighting in Marea, on the outskirts of Aleppo.
The group did not say who used the outlawed chemical. But activists and other rebel groups blame Islamic State, which is also fighting to topple the Syrian government.
The United States suspects Islamic State has used mustard gas in the past.
"We know that ISIL [Islamic State] has stated its intent to use chemical weapons and other terror weapons," U.S. Central Command spokesman Colonel Patrick Ryder said Friday. "It is another reason this group needs to be stopped as quickly as possible."
The Organization will turn over its full report to the United Nations.
Mustard gas was first used in World War I and has since been outlawed. It is a particularly vicious poison that burns the eyes, skin, lungs, and other organs.