The US Department of Homeland Security has warned airlines that terrorists could try to hide explosives in shoes.
It is the second time in less than three weeks that the government has issued a warning about possible attempts to smuggle explosives on a commercial jetliner.
Homeland Security said it regularly shares relevant information with domestic and international partners, but it declined to discuss specifics of a warning sent to airlines.
"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," the department said.
A US intelligence official said DHS released a notice to airlines reiterating that liquids, shoes and certain cosmetics were of concern, all of which are covered under existing Transportation Security Administration (TSA) security policies.
The latest warning was focused on flights headed to the US from abroad.
The official said "something caused DHS concern, but it's a very low threshold to trigger a warning like this".
Earlier this month Homeland Security warned airlines with flights to Russia to be on the lookout for explosive devices possibly hidden inside toothpaste.
TSA then banned passengers from bringing any liquids in their carry-on luggage on non-stop flights from the US 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.
Air passengers in the US have had to take off their shoes at airport security checkpoints since shortly after Richard Reid tried to ignite explosives hidden in his shoes on a Miami-bound flight in late 2001.
Reid pleaded guilty to terrorism charges and is serving a life sentence.
The travelling public has grown increasingly impatient with expanding security checks at airports.
TSA in recent years has changed some security procedures to allow young children and passengers 75 and older to keep their shoes on.
The security agency has also launched a fee-based programme that allows willing flyers to submit to background checks and avoid having to remove their shoes, jackets and small amounts of liquids packed in carry-on luggage.