WASHINGTON (AP) — The Obama administration has concluded that Syrian President Bashar Assad’s regime has used chemical weapons against the opposition seeking to overthrow him, U.S. officials said Thursday, crossing what President Barack Obama has called a ‘red line’ that would trigger greater American involvement in the crisis.

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.