Three hospitals near Damascus reported 355 deaths after an alleged chemical attack last Wednesday out of around 3,600 admissions with nerve gas-type symptoms, the medical charity Medecins Sans Frontieres (MSF) said on Saturday.
The Syrian opposition has accused government forces of gassing hundreds of people on Wednesday by firing rockets that released deadly fumes into rebel-held Damascus suburbs, killing men, women and children as they slept.
Opposition estimates for the death toll have ranged from 500 to well over double that number, but, with UN observers unable to visit the site, there has been no independent verification.
MSF has no staff of its own in the Damascus region, but has been supporting hospitals and medical networks there since 2012.
"The reported symptoms of the patients, in addition to the epidemiological pattern of the events - characterised 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," MSF director of operations Bart Janssens said in a statement.
"This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons."
Janssens said MSF could not confirm the cause of the symptoms or say who was responsible for the attack, but that it had sent 7,000 vials of atropine - an antidote against nerve agents.