US security officials warned about a potential shoe-bomb threat on international flights to the United States.
The Department of Homeland Security issued a warning to airlines based on "very recent intelligence" considered credible that assailants would try to attack passenger jets using explosives hidden in shoes.
Officials told NBC television that passengers may be subjected to enhanced security screenings and airlines will pay more attention to passengers' shoes.
Some airports already require passengers to take off their shoes for security after Richard Reid tried to detonate explosives stashed in his shoes during a Paris-Miami flight just three months after the 11 September 2001 attacks.
Terror groups are working on new shoe-bomb designs, according to intelligence collected by the United States and other allies, CNN reported.
Officials told NBC the information about the latest threat was not specific to any particular country, airline or time, stressing that there was no indication of a specific plot.
Homeland Security officials, who declined to comment specifically about the threat, told CBS television the alert was put out due to an "abundance of caution."
Earlier this month, US security officials warned of explosives hidden in toothpaste tubes.
"Our security apparatus includes a number of measures, both seen and unseen, informed by the latest intelligence and as always DHS continues to adjust security measures to fit an ever- evolving threat environment," the agency said.
On Christmas Day 2009, Nigerian man Umar Farouk Abdulmutallab tried to blow up an airliner as it made its descent into Detroit from Amsterdam, using explosives hidden in his underwear.