The United States and Britain have said that they have detected sarin, a nerve agent, in physiological samples from Syria and that it was probably used by government troops. French officials have gone further, saying there is “no doubt” that the Syrian government used sarin in at least one attack and possibly others.

But none of the evidence has been made public, and many experts on chemical weapons say that it is important to remain skeptical, that the anecdotal evidence that has emerged is inconclusive and needs to be investigated by an impartial organization. Some experts have been mystified by the relatively low number of deaths, given the toxicity of a nerve agent like sarin. They are also confused by the range of symptoms seen in videos disseminated by Syrian opposition activists — including some that seem mild — leading to questions about what kind of toxins were used, but also the veracity of some of the videos. …

Adding to the uncertainty, some experts said, is the incentive that President Obama may have unintentionally provided to exaggerate the reports. Last August the president said that the use of chemical weapons by the Syrian government would cross a “red line” and “change my calculus” on whether the United States should intervene in Syria — which is exactly what many of Mr. Assad’s opponents have hoped for.