"It is unlikely that any opposition group could have a capability to fabricate the large volume of information that is available publicly," the official said.
The official added the United States will strike again if the recent respond does not succeed.
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"We will continue to work with our allies, allies beyond those we took the strike with last night, to determine the best set of economic, political and diplomatic measures to ensure not just the Syrian regime, but all countries around the world … understand that chemical weapons use will not be tolerated," the official said. "At the same time we will continue to keep viable military options on the table."
The United States continues to work strongly with the United Nations members including Russia to move forward with the political process in Syria, another senior US administration official said.
A senior US administration official added that the United States wants to have an engagement with Russia that leads to greater peace and stability in the world.
Damascus has rejected all the allegations, noting that all chemical stockpiles were destroyed, which was confirmed by the OPCW. No evidence of Damascus' involvement in the reported chemical incident was presented, with a probe due to start on April 14.