ISTANBUL — The Islamic State may have used chemical agents in an attack against civilians and rival insurgents in northern Syria late last week, according to local rebels and an international aid group.

The assault on Friday in the city of Marea involved more than 50 shells and was centered on civilian areas, the Syrian American Medical Society, a humanitarian group, reported.

After the attack, the group’s field hospital received more than 50 patients, 23 of whom, including some children, showed symptoms of chemical exposure, including coughing, vomiting, wheezing and severe itching. Some also had blisters associated with mustard gas, the society said in a statement.

The report was corroborated by local rebel forces, who claimed that shells had been fired from Isnibil, a village east of Marea that is controlled by the Islamic State, also known as ISIS or ISIL.