A top UN envoy was in Damascus on Saturday to press for an investigation into an alleged chemical attack, as Syrian hospitals were reported to have treated thousands of "neurotoxic" cases.
US President Barack Obama, meanwhile, met his top national security advisers to weigh the response to the alleged massacre.
He is under mounting pressure to act following Wednesday's reported chemical attack near Damascus that opposition groups say was carried out by President Bashar al-Assad's forces and had killed as many as 1,300 people.
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.
"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," a White House official said.
Doctors Without Borders (MSF) said that 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.
The victims all arrived within less than three hours of each other, and MSF director of operations Bart Janssens said the pattern of events and the reported symptoms "strongly indicate mass exposure to a 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.
His comments came as UN Under Secretary General Angela Kane was in Damascus, tasked by Secretary General Ban Ki-moon with establishing the terms of an inquiry.
Ban is determined to "conduct a thorough, impartial and prompt investigation" into the chemical attack claims, his spokesman said.
French Foreign Minister Laurent Fabius, during a visit to the West Bank on Saturday, blamed Syria for a "chemical massacre" and said that "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.
On Friday, Syria's main opposition National Coalition pledged to guarantee the safety of the inspectors but warned that the "clock is ticking" before alleged evidence vanishes.
Coalition chief Ahmad al-Jarba was scheduled to hold a news conference on the issue in Istanbul later on Saturday.
Syria has yet to say if it will let the UN experts -- on the ground in Syria since August 18 to probe three other sites -- to inspect the latest allegations.
Russia urged Damascus to cooperate with the UN but dismissed calls for use of force against its ally.
In statements published on Saturday, German Chancellor Angela Merkel accused Russia and China of having blocked a UN text demanding the inspectors be given unfettered access.
One year ago, Obama warned the use of chemical weapons in Syria would cross a "red line" and have "enormous consequences".
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.
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 was surrounding 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 National Coalition denied that rebels had resorted to the use of 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.