The US has said that Syria has likely used chemical weapons against rebel forces on a "small scale," but emphasised intelligence services were still not 100 percent sure.
US spy agencies have investigated reports from Syrian opposition groups that President Bashar al-Assad's forces have used sarin gas on at least two occasions during the two-year-old conflict.
"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria," Caitlin Hayden, a US National Security Council spokesperson, said on Thursday.
The assessment, which she said was based in part on "physiological samples", points to the possible use of sarin, a man-made nerve agent used in two attacks in Japan in the 1990s.
It can cause convulsions, respiratory failure and death.
Hayden however warned the chain of custody of the weapons was "not clear, so we cannot confirm how the exposure occurred and under what conditions".