Passengers heading to the United States from Vancouver are among those who could face long delays at the airport. (CBC)

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.