Médecins Sans Frontières (Doctors Without Borders) said Saturday that three hospitals it supports in Damascus Province received about 3,600 patients with neurotoxic symptoms in less than three hours on Wednesday morning of Wednesday, after regime attacks — allegedly with chemical weapons — near Damascus.

MSF said that the hospitals said 355 of those patients died.

“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, MSF director of operations.

The hospitals told MSF that patients were treated using MSF-supplied atropine, a drug used to treat neurotoxic symptoms. MSF said that it is now trying to replenish the hospitals’ empty stocks of atropine and provide additional medical supplies and guidance.

MSF’s Dr. Janssens said that while it can neither scientifically confirm the cause of these symptoms nor establish who was responsible for the attack, “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.”

Janssens added: “This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons.”