The officials said earlier intelligence assessments that indicated Assad likely used such weapons had now been corroborated. However, the officials said the administration has not determined how it will respond. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly ahead of an expected White House announcement of the finding.
Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
Congress was also being notified of the chemical weapons determination on Thursday in classified documents sent to Capitol Hill, the officials said. The finding was aided by evidence sent to the United States by France, which along with Britain, announced last week that they had determined that Assad's government had used chemical weapons in the two-year conflict.