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LONDON (Reuters) - 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.
They suspect an organophosphate agent, most likely sarin gas, was involved in Wednesday's 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.
With dozens of smart phone-filmed videos spreading across the Internet within hours of the attack, the scale clearly outstrips anything since Saddam Hussein's chemical attack on the Kurdish town of Halabja in 1988 killed more than 3,000.
Rebels say between 500 and well over 1,000 people died in the pre-dawn strikes. However, experts say a relatively large number of survivors suggested Syria's most potent chemical weapon, VX gas, was not used. One droplet of VX can kill and the area potentially rendered lethal for much longer afterwards.
The region attacked - urban and semi-built-up areas in the Ghouta area largely east of Damascus - has been a center of resistance to Assad.
"It's been an area where there has been relative military deadlock and so, in that sense, it might make sense for Assad to use chemical weapons as part of a wider offensive," said Firas Abi Ali, Middle East analyst at IHS Country Risk.
In Paris, a spokesman for the Free Syrian Army said the rebels believed government forces had fired 29 missiles from three military bases in the Damascus area, striking many targets in the suburbs.
Not all of the missiles appeared to have carried chemical warheads, the FSA spokesman said, but those that did were suspected to have contained sarin, a Russian-made nerve agent called SC3 and liquid ammonia supplied by Iran.
Some analysts and Western officials say the rebels themselves have captured some basic chemical weaponry, but doubt they would have the capability to mount an attack on this scale.
Nerve agents such as sarin break down quickly in the environment, and it is far from clear whether local medical staff and activists were able to take properly uncontaminated samples and keep them refrigerated and in good condition.
U.N. Secretary-General Ban Ki-moon said on Friday he intends to conduct a "thorough, impartial and prompt investigation" into the latest alleged attack.
Most of the documented samples from previous suspected chemical attacks elsewhere in Syria, experts say, have themselves often been too poor for the specific chemical agent to be identified or proven.
"Something has killed a lot of people," says Dan Kaszeta, a former U.S. Army chemical officer and Department of Homeland Security expert who is now a private consultant. "We're not going to know exactly what until someone gets a sample."
Among the dozens of video clips of corpses and affected adults and children are a handful of pictures purporting to show the rockets themselves.
Relatively basic and with crude stabilizing fins, foreign chemical weapons experts say they bear a striking resemblance to devices found elsewhere in Syria in the aftermath of much smaller suspected attacks.
Capable of carrying perhaps 1-2 liters of chemical agent apiece, experts say they were most likely fired by some form of multi-barrel rocket launcher, or perhaps a large mortar.
Verifying the handful of warhead pictures from the Damascus area, however, is difficult, with the possibility they might be faked or reproduced from previous attacks elsewhere.
Some analysts say they doubt the pictured rockets could have caused the alleged level of casualties. That might suggest the use of a larger weapon such as a Scud ballistic missile.
"The size of the rocket is a big deal," says Steve Johnson, a former British Army chemical warfare expert and now a visiting fellow at the forensics Department of Cranfield University. "What we need is verified casualties figure, ideally from a neutral source such as the U.N."
Additional reporting by Kate Kelland and John Irish in Paris; editing by David Stamp