The U.S. could hit Syria with three days of missile strikes, perhaps beginning Thursday, in an attack meant more to send a message to Syrian President Bashar al-Assad than to topple him or cripple his military, senior U.S. officials told NBC News on Tuesday.

The State Department fed the growing drumbeat around the world for a military response to Syria's suspected use of chemical weapons against rebels Aug. 21 near Damascus, saying that while the U.S. intelligence community would release a formal assessment within the week, it was already "crystal clear" that Assad's government was responsible.

Vice President Joe Biden went even further, bluntly telling an American Legion audience in Houston: "Chemical weapons have been used."