A top UN envoy was in Damascus today 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 today, 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.
