U.S. homeland security officials are warning airlines flying to Russia to watch out for toothpaste tubes that may be filled with bomb-making materials.
The officials told U.S. news agencies there is no specific threat at this time, and they gave no details on the intelligence that prompted the warnings.
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.