Medecins Sans Frontieres reports hospitals it supports in Syria treated about 3,600 patients with “neurotoxic symptoms”, of whom 355 have died.
MSF said the patients had arrived in three hospitals in the Damascus governorate on August 21 – when opposition activists say chemical attacks were launched against rebels.
But MSF says it cannot “scientifically confirm” the use of chemical weapons.
Both sides in the conflict accuse each other of using them.
MSF says staff at the hospitals described a large number of patients arriving in the space of less than three hours with symptoms including convulsions, extreme salivation, contracted pupils and sight and respiratory problems.
The charity said many were treated with atropine, a drug administered to those with “neurotoxic symptoms”.
“MSF can neither scientifically confirm the cause of these symptoms nor establish who is responsible for the attack,” said MSF Director of Operations Bart Janssens.
“However, the reported symptoms of the patients, in addition to the epidemiological pattern of the events, characterized by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers, strongly indicate mass exposure to a neurotoxic agent.
“This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons.”
The opposition Syrian Observatory for Human Rights has given its latest assessment of the number of casualties from the alleged attacks.
The British-based group said it estimated that 322 had died, 54 of them children.
In the immediate aftermath, casualty figures varied widely with opposition activists saying between several hundred and more than 1,000 had been killed.
MSF’s disclosure adds to mounting allegations that chemical weapons were used in suburbs to the east of Damascus and in an area to the south-west on August 21.
Unverified video footage posted soon afterwards shows civilians, many of them children, dead or suffering from what appear to be horrific symptoms consistent with a chemical attack.
Rebels and opposition activists accuse forces loyal to President Bashar al-Assad of carrying out such attacks.
But state TV accuses the rebels, saying barrels of chemical weapons were found as troops entered previously rebel-held districts.
Soldiers had “suffocated” as they tried to enter Jobar, one of the towns in the Ghouta district around Damascus.