The Department of Homeland Security (DHS) has warned international flights flying into US from abroad of a possible shoe bomb threat.
Sources familiar with the matter reportedly said that the warning is not specific to an airline or time frame, and is based on very recent intelligence, but not related to domestic or outbound flights.
A statement from the DHS said: "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."
According to intelligence collected by the US and other allies, terror groups are working on new shoe-bomb designs that include cosmetics and liquids, CNN reported.
The bulletin issued by DHS called on screeners for enhanced security screenings of passengers' shoes.
Earlier this month ahead of Sochi Olympics, Homeland Security warned airlines flying to Russia about explosive devices possibly hidden inside toothpaste.
Some airports in the US have already tightened airline security after Richard Reid tried to ignite explosives in his shoes on a flight from Paris to Miami in late 2001.