The Yemen-based al-Awlaki has been linked to the Fort Hood massacre and the attempt in December to blow up a Detroit-bound jet by a man wearing explosives in his underpants.
The decision to add him to the US hit list required a National Security Council review because of his citizenship.
Members of the George W Bush administration have said they believe it never approved any American for a targeted killing.
However, officials now argue privately that Americans who side with the country's enemies are not ultimately "entitled to special protections"....
