In recent months the Obama administration has acknowledged at least two instances of Syrian chemical weapons stockpiles being moved but has said it believes those were efforts to put the material in more secure locations as fighting increased.

"We don't know if there is actual intent," the official said about the latest observed activities. "This is worrisome. This is a step beyond moving them around."

The official said if the weapons are used it could mean the red line articulated by President Barack Obama has been breached. If they are used, "this puts us into the contingency of potential U.S. action," the official said