While much remains sketchy about the apparent gassing of Syrians on the outskirts of Damascus, Western experts believe rockets or missiles were used to disperse a nerve agent in the worst chemical attack in a quarter of a century, Reuters said.

They suspect an organophosphate agent, most likely sarin gas, was involved in Wednesday August 21 attack. However, the basic chemical agent may have been mixed with other substances acting as preservatives and perhaps also to alter or add to the effects of the gas.

"Because they are non persistent agents, they dissipate very quickly," said Hamish de Bretton-Gordon, a former head of Britain's military counter-nuclear, biological and chemical warfare force and now a private contractor.

"In pure military terms, the idea is to drop these things on a population, kill lots of people very quickly, and then your own forces can go in without suffering consequences."

On Friday, U.S. President Barack Obama called for a full investigation by United Nations monitors already in Syria. Within Western governments, however, officials say there is little or no doubt that forces loyal to President Bashar al-Assad launched the strike on rebel-held areas.

Syria's government refutes the charge, while Syrian and Russian media have accused the rebels of staging the incident.