A medical group reported that those injured after at least 50 mortar shells were fired at residential areas of the town of Marea on Friday were exhibiting symptoms of chemical exposure, Al Jazeera reported on Monday.
Local sources told the Syrian American Medical Society (SAMS), a non-profit humanitarian organization, that ISIL terrorists engaged in clashes with militants had carried out the attack.
SAMS said one of its field hospitals in Aleppo received injured civilians with symptoms including respiratory irritation, wheezing, coughing, irritation and redness of the eyes and mucous membranes, skin irritation, and severe itching.
Roughly 30 civilians developed skin blisters, with doctors identifying the cause to be mustard gas, SAMS reported. No deaths have been reported as of yet.
Mustard gas, also known as sulfur mustard, is a chemical compound which has been used as a chemical weapon during World War I.
The targeting of civilians and the use of mustard gas are violations of international humanitarian law as per the Geneva Convention and Chemical Weapons Convention.
SAMS said its volunteers had taken samples from patient blood, clothing, and hair from the shelling site to be assessed.
Marea is on the frontline of fighting between ISIL and the so called Islamic Front group, an offshoot of the the Free Syrian Army militants comprised of almost a dozen factions including Ahrar al-Sham and al-Tawahid brigade.
The town is located along the highway linking Aleppo to the Turkish border, which makes it significant for ISIL as to being a way to transport supplies and bring fighters.
The US previously accused ISIL of using mustard gas in July against Kurdish forces in northern Syria, in addition to two attacks in August against Iraqi Kurdish forces.