Three experts said Thursday that claims by Syrian rebels and activists that President Bashar Assad's forces had used chemical weapons were credible based on video footage of the victims.
The U.S. has led international calls for United Nations inspectors to be allowed to immediately investigate Wednesday's incident in which 1,200 people were said to have been killed.
Hamish de Bretton-Gordon, a former commander of British counter-terrorist forces, and Gwyn Winfield, editor of Chemical, Biological, Radiological, Nuclear Explosive magazine, both said that graphic videos of victims supported the rebel account.
However, both experts said it would be incredibly hard to pinpoint exactly what chemicals had been used without testing on the ground.
The footage showed children choking and vomiting and adults writhing in agony.
“Having difficulty breathing and going floppy, a few tremors…. What a nerve agent does is it destroys your nerves, your body packs up,” Bretton-Gordon said. “Some of these are symptoms that you would expect to see from chemical weapons.”
He said that it was clear that the dying and injured people seen in the clips had not been killed by a conventional weapon.
“It is difficult to attribute to anything but an unconventional asymmetric weapon and at the moment the most likely...would appear to be a chemical weapon,” he said.
“I think that when you see the dead and dying children and the dead and dying babies with the symptoms that one would anticipate from some sort of chemical weapon, I don't think they could fake that,” he said.
Bretton-Gordon added that it was unlikely that an attack on this scale could have been delivered by an improvised device.
“They've been delivered by a platform that is designed to deliver chemical weapons, probably rocketry,” he said.
“This is very sophisticated weaponry, this is very sophisticated science, you can't sort of knock it up in your back room and deliver it. I'm not aware that the rebels have that kind of weaponry, so at the moment until we know more, it is looking like it is a regime-delivered massacre,” he added.
Winfield said that it would be impossible for an agent like sarin nerve gas to kill 1,200 people, but he added “certainly the signs and symptoms suggest that a chemical was used."
He said the available information pointed "to something that may well be a Syrian home brew, that is either a lower-toxicity sarin or alternatively the agent has been adulterated with other things."
Pure sarin, he explained, is a member of the “super toxic” chemical group, which initially is absorbed into a victim’s body, but leaves after death and disperses into the atmosphere.
Known as “off gassing” by experts, this process can affect people not caught up in the initial attack, including those trying to treat the sick and injured.
“If you look at the way they have been stacking fatalities in their temporary morgues, you wouldn’t be able to go in there without high degrees of respiratory, and possibly even skin, protection because you are touching those people,” he said.
“So the fact that they are able to operate in that environment without that equipment suggests that it is not a super toxic,” he added.
U.S. chemical weapons expert Dan Kaszeta told NBC News' U.K. partner ITV News that occasionally new chemicals were used.
"In Russia some years ago during a hostage situation a fentanyl gas, which is an opiate-based narcotic, was used and that was completely new to the people in my community. We'd never heard of it before, so occasionally new things do pop up," he said.
But he added that it was quite possible that it could have been one of many thousands of toxic and industrial chemicals that are used in commerce and industry, but haven't been used in the past for chemical warfare.
All three experts agreed that it was vital that the scene of the attack was investigated by a United Nations inspection team led by Ake Sellstrom, a Swedish biological and chemical weapons expert.
The investigators were recently allowed into the country but the probe is limited to three sites and only seeks to determine whether chemical agents were used, not who used them.
Reuters contributed to this report.
First published August 22 2013, 9:16 AM