The warning came a day after US officials said Anwar Al-Awlaki, identified as "chief of external operations" for Al-Qaeda's Yemen branch, was killed in an attack by missiles fired from multiple CIA drones in Yemen.

"The death of Awlaki, in the near term, could provide motivation for anti-American attacks worldwide from individuals or groups seeking to retaliate against US citizens or interests because of this action," the State Department said.