"They have a history of lies -- Iraq," he said.

Vice President Joe Biden said there was no question that Assad was responsible for the attack -- the highest-ranking U.S. official to say so -- and the White House dismissed as "fanciful" the notion that anyone other than Assad could be to blame.

"Suggestions that there's any doubt about who's responsible for this are as preposterous as a suggestion that the attack did not occur," spokesman Jay Carney said.

A U.S. official said some of the evidence includes signals intelligence -- information gathered from intercepted communications. The U.S. assessment is also based on the number of reported victims, the symptoms of those injured or killed, and witness accounts. The officials insisted on anonymity because they were not authorized to publicly discuss the internal deliberations.