The organization, also known as the White Helmets, and the Syrian American Medical Society, a medical relief organization, did not identify the substance used but said survivors treated at clinics smelled strongly of chlorine.
Those reports could not be independently verified because of a government blockade around the town.
Hours after the attack, the Army of Islam rebel group agreed to surrender the town and evacuate their fighters to rebel-held northern Syria, Syrian state media reported. The group also agreed to give up its prisoners, a key government demand.
The government agreed to halt its assault after three days of indiscriminate air and ground attacks.
More than 100 buses entered the town on Sunday night to take the fighters and their families to Jarablus, a town under the shared control of rebels and Turkey, said Syrian state-affiliated al-Ikhbariya TV.
Syrian state TV said two buses left early Monday and that 11 more buses were getting ready to move.
Syrian state media said dozens of civilians who had been held for years by the rebels were set free. The SANA news agency said the people were freed around midnight on Sunday, adding that they had been held by the Army of Islam group since 2013.
