US intelligence agencies believe "with varying degrees of confidence" that Syria's regime has used chemical weapons on a small scale.
The disclosure was made by the White House in a letter to Congress and by US Defence Secretary Chuck Hagel to reporters.
The White House has warned chemical weapons use would be a "red line" for possible intervention, but said the intelligence does not represent proof.
It added that President Barack Obama needed "credible and corroborated" facts before acting on that assessment.
During a visit to the United Arab Emirates, Mr Hagel said it was suspected sarin gas has been used against rebel fighters.
"The US intelligence community assesses, with some degree of varying confidence, that the Syrian regime has used chemical weapons on a small scale in Syria, specifically, the chemical agent sarin.
"As I've said, the intelligence community has been assessing information for some time on this issue and the decision to reach this conclusion was made in the last 24 hours."
Mr Hagel said while the United States could not confirm the origin of the chemical weapons, it believes any use would very likely have originated with the regime of President Bashar al-Assad.
The White House said the assessment that Syria's regime had used chemical weapons was based in part on physiological samples.