But officials cautioned that assessment is made "with varying degree of confidence" and "credible and corroborated facts" are still needed.
President Obama has said use of the chemical weapons by Assad, or any move to give them to terrorists, would cross a "red line," but didn't specify the consequences.
"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," Miguel Rodriguez, director of the White House office of legislative affairs, said in a letter to U.S. senators.
The administration cautioned that U.S. intelligence still has to "build on these intelligence assessments as we seek to establish credible and corroborated facts," The Hill newspaper reported, indicating no military action is yet planned.
Given the assessment, "it's up to the commander in chief, but something has to be done," said Sen. Dick Durbin, D-Ill., the Senate majority whip.
He was one of senators from both parties who called for action.
"The president of the United States said that if Bashar Assad used chemical weapons it would be a game changer, that it would cross a red line," Sen. John McCain, R-Ariz., said. "I think it's pretty obvious that red line has been crossed."
McCain said he wants the United States to create a safe zone for the Syrian opposition, set up a no-fly zone and gives weapons to rebel groups who are not terrorists, The Hill reported.