Footage from Douma in the aftermath of the attack appeared to show people using water to counteract the effects: AFP
The World Health Organization (WHO) has demanded "unhindered access" to Douma in Syria, to check reports from its partners that 500 people have been affected by a chemical attack there.
The Syrian government denies being behind any use of chemical weapons, reports BBC.
The US has threatened a "forceful" response to reports of an attack but Russia has called this a "pretext" to attack its ally, Syria.
The WHO cited reports that 70 people had been killed.
It said its "health cluster" partners reported that 500 people had suffered symptoms consistent with exposure to toxic chemicals, including difficulty breathing, irritation of mucous membranes, and disruption to the central nervous system.
"Two health facilities were also reportedly affected by these attacks," the WHO statement continued.
"We should all be outraged at these horrific reports and images from Douma," said Dr. Peter Salama of the WHO.
"WHO demands immediate unhindered access to the area to provide care to those affected, to assess the health impacts, and to deliver a comprehensive public health response."