Obama's talk about the use of chemical weapons crossing a "red line" appears to have put him into a corner, Hay said. "It's difficult to know how he's going to back out of that one, without some kind of punitive strike."
Hay said he hoped any military action would not be against al-Assad's chemical weapons stockpiles because "that runs the risk of spreading these agents even further."
The alleged use of chemical weapons in the August 21 attack in a suburb of Damascus was only the latest of more than a dozen such reports in recent months, according to Jon Day, the chairman of Britain's Joint Intelligence Committee.
"We have assessed previously that the Syrian regime used lethal CW (chemical weapons) on 14 occasions from 2012," he said Thursday in a two-page report to Prime Minister David Cameron.
If what Day described as "a clear pattern of regime use" of chemical weapons has indeed been established, why did the latest report spark a more bellicose response from world powers?
Part of the answer lies in the scale of the event, in which hundreds of people are reported to have died, said Dr. Howard Hu, a consultant on chemical weapons for Physicians for Human Rights and dean of the Dalla Lana School of Public Health at the University of Toronto.
In addition, the evidence is strong, he said. "This time, there was enough videotaped evidence of victims subsequent to the attacks to provide a level of detail that allowed observers like myself to see signs that were consistent with an acute response to a nerve agent. And that level of specificity, I think, also increased the level of certainty and urgency to this."
Hu noted the August 24 report by Medecins Sans Frontieres, also known as Doctors Without Borders. It cited information from three hospitals it supports in Syria's Damascus governorate that said some 3,600 patients had arrived at one of the hospitals within a three-hour time span showing neurotoxic symptoms, and that 355 of them died.
"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," said Dr. Bart Janssens, MSF director of operations.
Secretary of State John Kerry said Friday that U.S. intelligence information found that 1,429 people were killed in the attack, including at least 426 children.
Patients were treated with atropine, a drug for neurotoxic symptoms.
Janssens said the group could neither confirm the cause of the symptoms nor identify who was responsible.
However, he added, "the reported symptoms of the patients, in addition to the epidemiological pattern of the events -- characterised by the massive influx of patients in a short period of time, the origin of the patients, and the contamination of medical and first aid workers -- strongly indicate mass exposure to a neurotoxic agent. This would constitute a violation of international humanitarian law, which absolutely prohibits the use of chemical and biological weapons."
Hu said that the reports of medical personnel winding up with some of the same symptoms suffered by their patients "is very consistent with a toxic CW agent causing secondary effects. So, put it all together, it's a very, very compelling case."
Charles P. Blair, a senior fellow on state and non-state threats at the Federation of American Scientists, said Thursday that the August 21 report stands apart from the others in the number of reported casualties.
But questions remain about all of the reported cases -- as to what agents may have been involved and who was responsible, said Blair.
"I think it's likely the Assad regime has (used) sarin on potentially two occasions, maybe three that we know if, but it's done so using very, very small quantities of it," he told CNN in a telephone interview. "And it's used primarily to dislodge entrenched opposition forces and probably to test the international community" to see how it responds, he said.
That would not explain the apparently large use reported in the August 21 attack, which crossed the administration's oft-repeated "red line," he said.
"If it was a chemical attack, if it was the Assad regime, it was such an egregious and completely irrational use of chemical weapons that the administration had to respond," he said.
But that has left the administration in a dilemma, he said. "There's nobody on the ground right now that they can identify that they can back safely without the risk of that group being overrun or taken over by the jihadist groups in Syria."