WASHINGTON, Dec. 31 The American authorities in the last week directed a United States-bound flight from Mexico to turn around in midair and imposed extraordinary security measures on at least six other incoming flights because of terrorist concerns, federal officials said Wednesday. Officials were so concerned about possible attacks on at least five foreign flights that landed in the United States, including one on Wednesday night at Dulles International Airport outside Washington, that they moved the planes away from the main terminals and rescreened the passengers.