Some 355 people who showed "neurotoxic symptoms" died following the suspected chemical weapons attack this week near Syria's capital, Doctors Without Borders says.
The Paris-based humanitarian aid group said that three hospitals it supports in the Damascus region reported receiving roughly 3,600 patients who showed such symptoms over less than three hours on Wednesday.
A debate has ensued about who was behind the alleged gas attack on rebel-held Damascus suburbs that activists previously said killed more than 130 people. The attack has spurred demands for an independent investigation and renewed talk of potential international military action, if chemical weapons were indeed used.
Anti-government activists accuse the Syrian government of carrying out the toxic gas attack on the eastern suburbs of Damascus and have reported death tolls ranging from 136 to 1,300.
Meanwhile Syrian state media has accused rebels of using chemical arms against government troops trying to storm a contested area of Damascus, claiming a major army offensive in recent days had forced the opposition fighters to resort to such weapons "as their last card."
State TV broadcast images of plastic jugs, gas masks, vials of an unspecified medication, explosives and other items that it said were seized from rebel hideouts. It did not, however, show any video of soldiers reportedly affected by toxic gas in the fighting in the Jobar neighbourhood of Damascus.
The claims could muddy the debate about who was responsible for an alleged gas attack on rebel-held suburbs of the capita.
Just hours before the state media reports, the UN disarmament chief arrived in Damascus to press President Bashar Assad's regime to allow UN experts to investigate Wednesday's alleged attack. The Assad regime has denied allegations that it was behind that attack, calling them "absolutely baseless" and suggesting they are an attempt to discredit the government.
The US, Britain, France and Russia have urged the Assad regime and the rebels fighting to overthrow him to co-operate with the United Nations and allow a team of experts already in Syria to look into the latest purported use of chemical agents.
The UN secretary-general dispatched Angela Kane, the high representative for disarmament affairs, to push for a speedy investigation. She did not speak to reporters upon her arrival in Damascus Saturday.