US Defense Secretary Chuck Hagel said Thursday that American intelligence believes "to some degree of varying confidence" that Syria has used chemical weapons.
During a visit to the United Arab Emirates, he said it was suspected that sarin gas had been used against rebels.
Hagel said the use was "small scale" and did not specify where or when it took place.
The White House has warned chemical weapons use would be a "red line" for possible intervention.
Speaking to reporters in Abu Dhabi on Thursday, Hagel said "our intelligence community does assess, with varying degrees of confidence, that the Syrian regime has used chemical weapons on a small scale in Syria, specifically, the chemical agent sarin".
"It violates every convention of warfare," he added.