The United States has warned airlines flying to Russia that terrorists may try to smuggle explosives on board planes in tubes of toothpaste.
Russian government forces are on high alert to head off possible militant attacks at the Winter Olympics which begin in Sochi on Friday.
There are widespread concerns about security at the Games after suicide bombers killed 34 people in the Russian city of Volgograd in December last year.
US media are quoting a security official who says that this advice has been given to airlines that have direct flights to Russia, including some that originate in the US.
It is not clear what kind of explosives they are warning about, but there has been recent experience of people trying to smuggle explosives on planes in shoes and underwear, the BBC reports.
The Department of Homeland Security has issued a statement, saying that it regularly shares relevant information with domestic and international partners and it is not aware of a specific threat to the US at the moment.
Olympic flame in Sochi
The Olympic flame has arrived in Sochi on Wednesday. It will spend the next few days in and around the city before being carried into the Fisht stadium to light the cauldron at the opening ceremony. The stadium is about 40km south of the city centre.
Flame carriers will to include UN Secretary General Ban Ki-moon and IOC president Thomas Bach, as well as luminaries of Russian sport.
Russian president Vladimir Putin assured Mr Bach on Tuesday that all guests would feel comfortable at the Games.
"The main task is to make the Sochi Games a celebration for all sport lovers in the world," he said.