The United States has issued a warning to airports and to some airlines flying to Russia for the Olympics to watch for toothpaste tubes that could hold ingredients to make a bomb on a plane, a senior US security official said.
The official on Thursday did not say whether any specific intelligence led to the warning, which was not issued to the public, according to Reuters news agency.
ABC News first reported the warning.
Russian forces are on high alert to head off possible attacks at the Winter Olympic Games, which begin on Friday in Sochi.
The Winter Olympics open Friday in Sochi, Russia. Russian President Vladimir Putin visited the Olympics village Wednesday and said that security remains a major concern.
Islamic extremists have threatened to disrupt the games. A jihadist group from Dagestan claimed responsibility for the two suicide attacks that killed 34 people in Volgograd late last year.
Russian media say an Islamic militant suspected of assisting the Volgograd suicide bombers has been killed. Reports say Dzhamaldin Mirzayev died in a shootout with police Wednesday at a house in Dagestan.
Russian authorities have spent an estimated $2 billion to shore up security in advance of the Sochi Olympics. Thousands of security personnel are patrolling what is described as a "ring of steel" around the Black Sea resort to prevent terrorist attacks.
US security officials said they were not aware of any specific threats to the United States now.
In 2009 a Nigerian man tried to set off an explosive hidden in his underwear aboard a US-bound airliner. In 2001 a man tried to blow up a jumbo jet with explosives in his shoes.