The U.S. Homeland Security Department has advised airlines to be on the lookout for potential bombs hidden in passengers' shoes, CNN reported on Wednesday.
Unidentified officials emphasized to the news channel that the warning is not tied to a specific plot. However, intelligence gathered by the United States and other nations suggests that extremist organizations have been trying to develop new shoe bombs, the sources said.
The DHS warning was for international flights bound for the United States.
"This threat is not specific or credible enough to require a specific response," an anonymous intelligence official said. "DHS often issues these alerts out of an abundance of caution, but this does not necessarily rise to the level of facilitating a response."
Most flight passengers in the United States are already required to remove their shoes for an X-ray scan before boarding flights.
A different intelligence insider said the government warning also covered makeup and liquids.
However, an official said the shoe-bomb concern was not connected to recent DHS warnings issued about explosive ingredients possibly being smuggled inside toothpaste containers and brought onto passenger flights bound for Russia. The Transportation Security Administration earlier this month issued a temporary ban on all liquids in carry-on bags for direct U.S. flights to Russia.