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.
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".
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."
"There is proof terrorist groups carried out this action," foreign ministry spokesman Abbas Araqchi said, without giving any details.
