The U.S. State Department's alert was issued Saturday after a successful CIA drone bombing attack that killed senior al-Qaida leader Anwar al-Awlaki Friday.
Awlaki and several others were burned beyond recognition, tribesman in the region told reporters. They said they would only release the bodies to immediate family members, CNN said.
Awlaki was born in the United States and fluent in English, which intelligence officials say led him to be a top recruiter for "homegrown terrorists."
His death led the State Department to caution U.S. citizens worldwide to be vigilant of "the potential for retaliation against U.S. citizens and interests" at least until Nov. 30.
The FBI and the Department of Homeland Security issued a similar joint caution hours after Awlaki's death was reported, focusing more on the potential of domestic attacks by "homegrown violent extremists."