Police in New York City are on alert for possible revenge attacks following the killing of Anwar al-Awalaki in Yemen.
The US State Department also issued a worldwide travel alert on Saturday warning of the possibility of anti-American attacks.
Awalaki was the leader of external operations for al-Qaeda in the Arabian Peninsula and was implicated in several plots to blow up US airliners.
He was killed by an American drone strike with several of his associates.
In a brief statement on Friday, the defence ministry in Yemen said Awlaki had been killed in Khashef in Jawf about 140km east of the capital, Sanaa.
The BBC says the killing is significant because Awlaki's use of modern media enabled Awlaki to reach out to and inspire people susceptible to radicalisation.