The department is deeming new intelligence credible enough to prompt the alert. Officials say there's no specific threat or plot, but it's acting out of caution.
The U.S. government is warning airlines to pay close attention to possible explosives hidden in shoes and cosmetics.
Shortly after the Sept. 11 attacks, passengers on an American Airlines flight stopped Richard Reid from detonating explosives in his shoes. He pleaded guilty and is serving a life sentence. The incident changed airline security, with most passengers having to remove shoes at security checkpoints.
Terrorism experts say that airlines continue to be a target of terrorists. Earlier this month, Homeland Security warned airlines with flights to Russia to be on the lookout for explosive devices possibly hidden inside toothpaste.
The Transportation Security Administration then banned passengers from bringing any liquids in their carry-on luggage on nonstop flights from the U.S. to Russia. That warning became public just days before the opening ceremonies of the Winter Olympics in Sochi.
It is unclear if the latest warning is related to the earlier threats to Russia-bound flights.
ABC News contributed to this report.
