WASHINGTON - US and allied intelligence agencies have made a preliminary assessment that Syrian government forces used chemical weapons to attack an area near Damascus this week and that the act likely had high-level approval from President Bashar Assad's government, according to US and European security sources.The sources, speaking on condition of anonymity, cautioned that the assessment is preliminary and, at this stage, they are still seeking conclusive proof, which could take days, weeks or longer to gather. 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, US 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 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.