U.S. struggling to determine whether Syria has used chemical weapons The Obama administration disclosed Thursday that the U.S. intelligence community believes with "varying degrees of confidence" that Assad's government has used the chemical agent sarin "on a small scale." U.S. officials declined to describe the nature of the evidence that U.S. spy agencies evaluated, except to say that it included "physiological samples," a term that likely refers to blood samples collected from victims of attacks. The lack of conclusive evidence has made a difficult policy dilemma even more complicated, particularly as U.S. allies, including Israel, Qatar and Britain, have made firmer statements about their interpretation of accounts, photographs and physical samples that are being analyzed. Jeffrey Lewis, a weapons expert at the James Martin Center for Nonproliferation Studies, said the Obama administration should refrain from declaring that its red line has been crossed, noting that exposure to sarin, a nerve agent that disrupts the muscles that control breathing and other functions, may have been accidental. "If we're wrong and he's still restraining himself, Assad may conclude that he's already paying the price of using chemical weapons," Lewis said. "What we don't want is for Assad to massacre thousands of civilians like Saddam did in Halabja," he added, referring to a Kurdish town in northern Iraq that was gassed in 1988. The first substantive reports suggesting the besieged Syrian state could be planning to use chemical weapons came last December, when Western intelligence agencies reported that Assad's government could be mixing precursors to load onto munitions. The Obama administration found the prospect so chilling that it warned Assad that the use of chemical or biological weapons would be a "red line" for the United States, although it has not spelled out how it would respond. Rebels have alleged that the regime has used chemical weapons in the western city of Homs and near Aleppo, a northern city that remains fiercely contested. Last month, rebels and Assad's government traded blame after an attack in Khan al-Asal, a town southwest of Aleppo where people reportedly vomited and struggled to breathe after a munition struck. That same day, similar accounts emerged from a small town east of Damascus that forces loyal to Assad have been struggling to recapture from rebel control. Jeffrey White, a Syria expert at the Washington Institute who worked as a Pentagon intelligence analyst for decades, said the timing of the reported incidents makes it unlikely that the exposure was accidental. "It's troubling," he said. Why use it in such a low-level tactical way? They could be testing our red lines. Even if sarin was used in an attack, finding proof in soil samples can be difficult unless tests are conducted immediately, experts say. Sarin breaks down rapidly when exposed to air and sunlight, and the residue left behind can be hard to distinguish from other chemical compounds, said David Kay, a U.S. scientist who led an investigation of Hussein's weapons program. Kay cited the faulty laboratory analysis - essentially, a false positive - that led to the 1998 U.S. missile attack on a pharmaceutical plant in Sudan. U.S. officials ordered the strike based on soil samples that falsely suggested that the plant was manufacturing sarin. "The farther you are from it, and the more time that passes, the less confidence you have in your results," said Kay, now a senior fellow at the Potomac Institute for Policy Studies, a Washington think tank. Syria called on the United Nations to investigate the use of chemical weapons after the Khan al-Asal incident, which happened March 19. That prompted Britain and France to urge the United Nations to conduct a more thorough investigation that would also examine reports of chemical weapons use in Homs and the town east of Damascus. Syria has resisted granting access to the U.N. team, insisting that any investigation be limited to Khan al-Asal. In a confidential letter, Britain has informed U.N. Secretary General Ban Ki-moon that it has obtained evidence that Syrian forces been hit by a projectile containing sarin in Khan al-Asal, possibly in a case of friendly fire. The Syrian army, Britain asserted, had fired a chemical shell at a public facility suspected of harboring opposition elements, but it veered off target, striking a Syrian government installation. Colum Lynch at the United Nations contributed to this report.