Three hospitals near Damascus reported 355 deaths last Wednesday out of around 3,600 admissions with nerve gas-type symptoms, the medical charity Medecins Sans Frontieres have said.
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.
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.
The MSF statement comes as the UN's top disarmament official Angela Kane arrived in Syria while state-controlled media claim troops have found chemicals in rebel tunnels.
She is seeking access to the site of Wednesday's apparent chemical weapons attack in Ghouta, east of Damascus, in which reports say over 1,000 civilians may have died.
UN inspectors already in the country investigating previous reported attacks have not been allowed visit the Damascus suburb where the atrocity occurred.
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."
Mr 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.
Iranian President Hassan Rouhani also acknowledged for the first time chemical weapons had killed people in ally Syria and called for the international community to prevent their use.
"Many of the innocent people of Syria have been injured and martyred by chemical agents and this is unfortunate," recently- elected Mr Rouhani was quoted by the ISNA news agency.
"We completely and strongly condemn the use of chemical weapons," he said, according to the agency.
Tehran has previously accused Syrian rebels of being behind what it called suspected chemical attacks.
US President Barack Obama has been meeting with his national security advisers today to discuss the alleged attack and the options available to the administration.
Syria's government denies using such weapons and Iran's foreign minister earlier this week said groups fighting Syrian president Bashar al-Assad in a rebellion since 2011 must have been behind what he then said was just a suspected attack.
Russia, another major ally in the Syrian government, has also blamed opposition forces.
Today's reports on Syrian state television sought to back this view, claiming some soldiers have been suffocating after the find in the suburb of Jobar in Damascus.
Syria's uprising against four decades of Assad family rule has turned into a civil war that has killed more than 100,000 people.
Foreign powers have said chemical weapons could change the calculations in terms of intervention and are urging the Syrian government to allow a UN team of experts to examine the site of Wednesday's reported attacks.
The US is repositioning naval forces in the Mediterranean to give Mr Obama the option for an armed strike on Syria, although officials cautioned that Mr Obama had made no decision on military action.