Around 3,600 Syrians have been treated for “neurotoxic symptoms”, and 355 of them have died, says the humanitarian group Doctors Without Borders (MSF).
It comes as Prime Minister David Cameron and US President Barack Obama discussed the situation in Syria in a telephone call.
Both leaders expressed their “grave concern” about the alleged use of chemical weapons outside the capital Damascus.
MSF said the victims flooded into three Syrian hospitals on the day that hundreds of people were apparently killed in a “chemical massacre”.
They all arrived within less than three hours of each other, said MSF director of operations Bart Janssens.
He said the pattern of events and reported symptoms “strongly indicate mass exposure to neurotoxic agent”.
“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,” he said.