US authorities have issued a warning to airlines flying to the United States to watch out for individuals who may have hidden bombs in their shoes.
The warning came from the Department of Homeland Security, government sources told the Reuters news agency late on Wednesday.
The sources, Reuters reported, said the warning principally applied to flights originating overseas and heading for the US, rather than domestic flights or planes headed overseas from the US.
However, the sources suggested the warnings did not mean that the US had specific intelligence indicating a plot to use a shoe bomb overseas was in progress.
One source said the alert was being issued "out of an abundance of caution", while a second said the alert was "not a big deal."
Wednesday's warning follows one earlier this month in which US authorities warned airlines flying to Russia for the Sochi Winter Olympics to watch out for toothpaste tubes that could hold bomb-making ingredients and could be smuggled through airport security.
Since the September 11, 2001 attacks on the US, there have been two known attempts to blow up US-bound airliners using bombs hidden in their clothes or footwear.
In December 2001, Richard Reid of Britain tried to set off explosives concealed in his shoe on a Paris-to-Miami American Airlines flight. Passengers subdued him before he could light the fuse.
And on Christmas Day 2009, Umar Farouk Abdulmutallab of Nigeria tried to detonate an explosive hidden in his underwear as his Delta Air Lines flight descended into Detroit after taking off from Amsterdam. Abdulmutallab's device malfunctioned, burning him, and he was subdued by passengers.
Both Reid and Abdulmutallab are serving long sentences in US prisons.