Thousands of patients were admitted to Damascus hospitals on Wednesday displaying signs of neurotoxicity, a Médecins sans Frontières spokesperson said on Saturday.
Though not conclusive, the revelation amounts to the first independent report that chemical weapons were deployed this week.
The charity, which works with several hospitals in the Syrian capital, said that more than 3,500 patients presented with symptoms of neurotoxicity within hours of the alleged attack. More than 355 have reportedly already succumbed to the exposure.
"Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress," said Dr Bart Janssens, director of operations at the charity.
He added: "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.
"This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons."
Shaam News Network image purporting to show mourners next to bodies of victims of an attack
The international community is already under pressure to act in the face of increasing treachery from the Assad regime, with William Hague refusing to rule out options for the West to prevent further bloodshed.
Speaking on Friday, the British Foreign Secretary said the UK will be ready to go back to the United Nations security council to secure a stronger mandate "for the world to speak together more forcefully about this" if there is no movement over the next few days.
"This is what we are focused on and we are working with countries all over the world to try to bring this about and to try to establish the truth to the satisfaction of the world about what is clearly a terrible atrocity.
"The only possible explanation of what we have been able to see is that it was a chemical attack and clearly many, many hundreds of people have been killed, some of the estimates are well over 1,000," he said.
"There is no other plausible explanation for casualties so intense in such a small area on this scale.
On Saturday, Chuck Hegel said the US military option in Syria was being weighed; with the Pentagon reportedly moving naval forces into position should President Obama order military action.
Reported by AP, Hegel said: "The Defense Department has a responsibility to provide the president with options for contingencies, and that requires positioning our forces, positioning our assets, to be able to carry out different options – whatever options the president might choose."