The United States is imposing even tighter controls on air travel, amid warnings from intelligence officials that al-Qaeda in Yemen is planning fresh attacks on airliners.
The measures include enhanced random screening and more air marshals on international flights.
US Homeland Security Secretary Janet Napolitano says al-Qaeda is a continued threat and a determined enemy.
The US has already boosted security following an attempted bombing of a trans-Atlantic jet on 25 December.
Umar Farouk Abdulmutallab, 23, of Nigeria, was overpowered by passengers on the plane.
He is accused of trying to detonate a bomb on a flight to Detroit and is charged with the attempted murder of 290 people. He has pleaded not guilty.
He caught the flight to Detroit from the Netherlands, after connecting from a flight from Nigeria.