Airlines are being warned to watch for explosives hidden in the shoes of passengers flying into the US from overseas.
The alert issued by the homeland security department on Wednesday was based on intelligence indicating that a shoe bomb may be used to blow up a United-States-bound plane. Officials said the threat was not specific to a particular airline, flight, country or time and was not related to the Winter Olympics being staged in Sochi, Russia.
The alert was issued "out of an abundance of caution", a homeland security official said.
Screeners at international airports were instructed to step up scrutiny of passengers boarding flights for the US. This will involve increased use of swabs that can detect traces of explosive powder on shoes, bags and hands. They also are likely to pull aside more passengers for pat-downs and for full-body screening, officials said.
In December 2001, three months after the September 11 attacks on the US, British national Richard Reid first made the world aware of such threats when the al-Qaeda operative boarded an American Airlines flight from Paris to Miami with explosives packed in his shoes.
Passengers and crew members subdued Reid when he attempted to ignite the explosives. He was arrested after the plane made an emergency landing at Logan International Airport in Boston.
Reid pleaded guilty to eight counts of criminal terrorism in federal court in 2002 and is serving a life sentence in prison.
Yesterday's alert followed the US government's February 6 ban on all liquids, gels and aerosols from carry-on luggage on flights between the US and Russia ahead of the Olympics. The restrictions were triggered by warnings that terrorists might hide bomb-making materials in toothpaste tubes for assembly into an explosive device during or after flights.
Onboard explosives have been a focus for US aviation security in the years since the September 2001 terror attacks.
The devices that were smuggled aboard the American jet by Reid contained the explosive known as by the acronym TATP and were powerful enough to rip a hole in the fuselage, US authorities said.
Additional reporting by Bloomberg