US authorities have issued a warning to airlines flying to the US to watch out for militants who may have hidden bombs in their shoes, US government sources said.

The warning came from the Department of Homeland Security, the sources said, and it is consistent with concerns security agencies have about militants trying to smuggle explosives onto airplanes in shoes, cosmetics or liquids.

The sources 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, some sources said 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." A second source 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, Islamist militants have made two known attempts to blow up US-bound airliners using bombs hidden in their clothes or footwear. - Reuters