All airlines flying to the United States or within it were told Thursday to prepare for even tighter security because of the al-Qaeda threat from Yemen, a law enforcement official said.
The U.S. increased the number of air marshals on international flights and pressed for more random screening at airports as intelligence officials warned that al-Qaeda's branch in Yemen was continuing to plot attacks on the United States.
A U.S. counterterrorism official said American intelligence agencies were intensely examining all information about threats from the Yemen-based al-Qaeda in the Arab Peninsula, including potential plots and specific individuals.
Counterterrorism agencies have serious concerns about al-Qaeda plots emanating from Yemen, the official said.
The officials did not pinpoint any specific evidence of new plots since the Christmas Day bombing attempt by a Nigerian national on a Detroit-bound flight. Both spoke on condition of anonymity because they were not authorized to discuss intelligence publicly.
Homeland Security Secretary Janet Napolitano said in a statement that travellers should allow for extra time, especially if flying into the U.S. from overseas, as officials stepped up security.
"We are facing a determined enemy and we appreciate the patience of all Americans and visitors to our country, and the co-operation of our international partners as well as a committed airline industry," she said.