International aid group Medecins Sans Frontieres said on Saturday medical facilities it funds had treated over 3,500 patients and tallied 355 deaths from Wednesday's attack in Damascus's eastern Ghouta suburb.
The group was wary of absolute pronunciations. "MSF can neither scientifically confirm the cause of these symptoms nor establish who is responsible for the attack," said MSF director of operations Bart Janssens.
But Western governments seemed unwilling to await the results of the investigation that they are pressing for.
The announcement of the agreement came shortly after a senior US official said there was "very little doubt" that a chemical weapon had been used by government forces.
And French President Francois Hollande claimed today that a "body of evidence" suggested that chemical weapons had been used and President Bashar Assad's government was most likely behind it.
On Saturday evening US President Barack Obama and British Prime Minister David Cameron agreed that there should be a "serious response" if Syrian troops had used chemical weapons.
The government and Syria's multitude of rebel groups have traded accusations of blame, with the government continued to deny any use of chemical weapons by its forces.
It warned the US not to launch military action against Damascus, with Information Minister Omran Zoabi saying that US intervention would bring chaos and that the Middle East would "burn."
Syria has alliances with Iran, Lebanese Hezbollah guerillas and Palestinian militant groups.
Violence has already spilled to Lebanon, Turkey and Iraq over the past year.
The country also borders Israel-occupied territory, making the fallout from military action unpredictable.
Mr Zoubi blamed the rebels for the chemical attacks, claiming that "the rockets were fired from their positions and fell on civilians."
Meanwhile, US naval units were moving closer to Syria and a fourth warship armed with ballistic missiles sailed into the eastern Mediterranean.