A report by the United Nations' chemical weapon monitoring agency confirms that deadly sulphur mustard, commonly known as mustard gas, was used in an attack on a northern Syrian town in August.

The Organization for the Prohibition of Chemical Weapons (OPCW) said Friday that the mustard gas was used during fighting between ISIS militants and a rebel group in the town of Marea, on the outskirts of Aleppo, according to Reuters. The chemical weapon attack left an infant dead.

"In this case, the team was able to confirm with utmost confidence that at least two people were exposed to sulphur mustard, and that it is very likely that the effects of this chemical weapon resulted in the death of an infant," the report said, AFP reported.

While the U.N. agency did not directly blame ISIS for using mustard gas, the U.S. and activists have pointed towards the Sunni jihadist organization as responsible for the attack.

"We know that ISIL [Islamic State] has stated its intent to use chemical weapons and other terror weapons. It is another reason this group needs to be stopped as quickly as possible," U.S. Central Command spokesman Colonel Patrick Ryder said Friday, according to VOA News.

Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, accused ISIS of using mustard gas during fighting in Marea.  "Daesh used toxic gases during its attack on Marea in August," he said, according to Aranews.

OPCW will formally hand over the report to U.N. Secretary General Ban Ki-moon this month.