Airlines flying into Russia for the Winter Olympics have been warned by the US that explosives hidden in toothpaste tubes could be smuggled on to aircraft.
The Department of Homeland Security told the BBC that the alert affected flights direct into Russia as the Games start today in Sochi.
The agency said it was not aware of any specific danger to the US at this time.
It is the latest reported threat against the region, which has strengthened security surrounding the Games.
The Department of Homeland Security said that it "regularly shares information with domestic and international partners, including those associated with international events such as the Sochi Olympics".
The White House National Security Council said the latest threat had not altered existing travel guidelines for Sochi.
"If we should receive information in the coming days and weeks that changes our assessment of whether people should travel to Sochi, we will make that information public," a spokeswoman told US media.
The US has also placed two warships in the Black Sea in case of a security breach during the games, which run until February 23.
Fears were raised following two suicide attacks in Volgograd in December, and numerous threats from Islamist militants in the Caucasus region.
Two failed attempts have been made in recent years to detonate explosives on passenger aircraft.
Briton Richard Reid tried to set off a bomb packed in his shoes on a Miami-bound flight in 2001, while Nigerian Umar Farouk Abdulmutallab attempted to trigger explosives hidden in his underwear on a plane to Detroit in 2009.