Syrian state media quickly denied that troops had deployed chemical weapons on Douma, the last opposition-held town in the battered Eastern Ghouta enclave.
Regime forces resumed a military blitz of Douma on Friday after an apparent breakdown in negotiations between government backer Moscow and Jaish al-Islam, the rebels that hold the town.
On Saturday, the Syrian Observatory for Human Rights monitoring group said air strikes on the northern edges of the town had left 11 people, including five children, "suffocating and suffering shortness of breath."
The monitor told AFP it could not specify the reason for the symptoms.
But the White Helmets rescue forces said Douma had been hit with toxic gas.
"Cases of suffocation between the civilians in the residential neighbourhood in the city Douma after it was targeted by poison gas chlorine," it wrote on its English-language Twitter account.
The Syrian American Medical Society (SAMS) also told AFP that medical staff they supported inside Douma had reported chlorine use.
"I spoke to one of the doctors inside the town, who told me... they received a number of wounded with symptoms from chlorine gas," said SAMS advocacy director Mohammad Katoub, based in Turkey.
Syria's regime has been accused of using toxic gas including chlorine and sarin throughout the seven-year conflict, but it has repeatedly denied the claims.
On Saturday, a government source told state media that the accusations were a "farce."
"The army, which is progressing rapidly and with determination, does not need to use any chemical substance," the source told state media.
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