The United States believes with varying degrees of confidence that Syria's regime has used chemical weapons on a small scale, the White House said on Thursday.
But it added that President Barack Obama needed "credible and corroborated" facts before acting on that assessment.
The disclosure, made by the White House in a letter to Congress and by U.S. Defense Secretary Chuck Hagel to reporters, moves the United States closer to declaring that Syria has crossed Obama's "red line" on some kind of deeper involvement in the country's civil war.
The White House has not specified what action Obama might take if he determines with certainty that Syria has crossed that red line with any chemical weapons use. But in its letter to lawmakers, it warned it was ready to respond.
"The administration is prepared for all contingencies so that we can respond appropriately to any confirmed use of chemical weapons, consistent with our national interest," Miguel Rodriguez, White House director of the office of legislative affairs, said in a letter to lawmakers.
The White House said the assessment that Syria's regime had used chemical weapons - specifically the chemical agent sarin - was based in part on physiological samples.
"The intelligence community has been assessing information for some time on this issue and the decision to reach this conclusion was made in the past 24 hours," Hagel told reporters in Abu Dhabi.
The White House stressed that, given the stakes involved, intelligence assessments alone were not sufficient. It said "facts that provide us with some degree of certainty will guide our decision making."
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.