Medecins Sans Frontieres (MSF, Doctors Without Borders) said 355 people had died earlier this week of "neurotoxic" symptoms as the Syrian government and its foes accused each other of using chemical weapons.
MSF said around 3,600 patients displaying "neurotoxic symptoms" had flooded into three Syrian hospitals on the day of the alleged attacks, and 355 of them 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," MSF director of operations Bart Janssens said.
But MSF stressed it had no scientific proof of the cause of the symptoms nor could it confirm who carried out the attack.
A senior UN envoy was meanwhile in Damascus to press for an investigation into the alleged chemical attack on Wednesday, as US President Barack Obama met his top national security advisers to weigh a possible response. He also discussed the crisis with British Prime Minister David Cameron.
Obama and Cameron expressed their grave concern Saturday about the "increasing signs" of a major chemical weapons attack.
A White House statement said the two leaders vowed during a telephone call to "continue to consult closely" regarding the alleged attack, as well as potential international responses.
But Cameron's Downing Street office went further, noting that the two leaders "are both gravely concerned by... the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people".
"The fact that President (Bashar) Assad has failed to cooperate with the UN suggests that the regime has something to hide," the British statement said, stressing that "significant use of chemical weapons would merit a serious response from the international community".
Australian Prime Minister Kevin Rudd said Sunday the indications "point strongly" to the use of chemical weapons and that it was up to Assad's regime to prove it had no hand in the attacks.
French Foreign Minister Laurent Fabius, during a visit to the West Bank on Saturday, blamed Syria for a "chemical massacre" and said "the Bashar regime is responsible".
But Damascus ally Iran blamed the rebels and warned the West against any military intervention.
"There is proof terrorist groups carried out this action," foreign ministry spokesman Abbas Araqchi said, without giving any details.
Warning against any Western military intervention in the conflict, Araqchi said "there is no international authorisation for" such action.
Obama is under mounting pressure to act following the reported attack near Damascus that opposition groups say was carried out by Assad's forces and had killed more than 1,000 people.
In Washington, a White House official said: "The president has directed the intelligence community to gather facts and evidence so that we can determine what occurred in Syria.
"Once we ascertain the facts, the president will make an informed decision about how to respond."
"The president also received a detailed review of a range of potential options he had requested be prepared for the United States and the international community to respond to the use of chemical weapons," a statement said.
The Syrian government has strongly denied the allegations but has yet to accede to demands that UN inspectors already in the country be allowed to visit the sites of the alleged attacks.
However, Syrian Foreign Minister Walid Muallem was quoted by his Iranian counterpart, Mohammad Javad Zarif, as saying Damascus would facilitate such a visit.
"The Syrian government will cooperate with the United Nations mission now in Syria to create the conditions for a visit to zones where terrorist groups have carried out attacks with chemical weapons," he was quoted as saying.
US Secretary of State John Kerry spoke Thursday with Muallem about the alleged attack.
Kerry discussed the attack "to make clear that if, as they claimed, the Syrian regime has nothing to hide, it should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence," a US official said.
Russia urged Damascus to cooperate with the UN but dismissed calls for use of force against its ally.
With the two sides trading accusations, UN Under Secretary General Angela Kane is in Damascus, tasked by Secretary General Ban Ki-moon with establishing the terms of an inquiry.
UN experts have been on the ground in Syria since last Sunday to probe three other sites.
Meanwhile, Information Minister Omran al-Zohbi said the regime had never used chemical weapons, "in any form whatsoever, be it liquid or gas".
For its part, the Syrian Observatory for Human Rights said more than 300 people had died from the effects of gas, including 82 women and 54 children.
The violence continued Saturday, with a watchdog accusing the regime of striking by air several rebel positions, including in Jobar, and reporting that insurgents seized a strategic town in the northwest.
State television said an army unit surrounded a "sector of Jobar where terrorists used chemical weapons", adding that soldiers who tried to enter the neighbourhood had "suffocated".
Rebels have "resorted to chemical weapons after the successes of the Syrian army in recent days", the television charged.
The opposition National Coalition denied that rebels had used chemical arms, saying the government was only trying to divert attention from its own use of them.
The "international community knows full well that the Assad regime is the only party in Syria which possesses the means to produce, use and stock chemical weapons", it said.
The United Nations says more than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, while millions more have fled the country or been internally displaced.