The pro-opposition Orient News claimed a child had been killed in the reported attack by a regime aircraft on the town of al Shifuniyah.
The Syrian Observatory for Human Rights monitor also reported the youngster’s death and said 13 people had suffered breathing difficulties.
Yaqub, a doctor who treated those affected in hospital, said he suspected “chemical weapons, probably a chlorine gas attack”.
He said a three-year-old had died of asphyxiation.
A US medical organisation said 16 patients in the hospital had symptoms indicating they were exposed to chemical compounds.
The Syrian American Medical Society said among those being treated with oxygen masks were six children and four women.
The Syrian regime, which has repeatedly denied using chemical weapons, has been accused of several chlorine gas attacks in recent weeks, including two in January in eastern Ghouta, on the edge of the capital Damascus.
Moscow, a staunch ally of President Bashar al Assad’s regime, accused his opponents of using “toxic substances” to make it appear that they had been deployed by regime forces.
It comes as fresh regime airstrikes and heavy clashes shook the rebel-held enclave despite a UN demand for a ceasefire.
On Saturday, the Security Council adopted a resolution calling for a 30-day truce in Syria “without delay” to allow for aid deliveries and medical evacuations.
At least 14 civilians, including three children, were killed in strikes on Sunday, said the Observatory, bringing the total number of dead in the week to at least 530, among them over 130 children.
Observatory chief Rami Abdel Rahman said there appeared be fewer airstrikes but that fighting had intensified on the ground.
Heavy clashes erupted in southern areas of eastern Ghouta, killing at least 13 members of pro-regime forces and six fighters from the Jaish al Islam rebel group, he said.
Syrian state news agency SANA said insurgents breached the truce by firing 15 shells on Sunday on government-held areas on the edge of Ghouta.
The two main rebel groups controlling the enclave – Jaish al Islam and Faylaq al Rahman – welcomed the Security Council demand, but vowed to fight back if there were renewed attacks.
UN diplomats say the resolution was watered down to ensure it was not vetoed by Russia, which has provided diplomatic and military support to the Assad regime.