The U.S. has banned passengers from carrying liquids, aerosols, gels and powders in their hand-luggage on direct flights to Russia during the Winter Olympics in Sochi.
On Thursday, the department issued a warning that terrorists could insert explosive devices into tubes of toothpaste, which could then be smuggled onto planes in their carry-on bags. These devices could then be exploded on route, or transported to Sochi.
Delta Air Lines later posted a Transportation Security Administration directive on its website, saying "no liquids, gels, aerosols and powders of any size" are to be permitted in the passenger cabin of the U.S.-Russia flight. Prescription medication is to be allowed onboard, however.
Carry-on liquids have been banned on all flights from Russian airports ahead of the Games.
Concerns over security have dominated the headlines in the run-up to the Sochi Olympics, given the venue's close proximity to the restive North Caucasus region and the two bombings in Volgograd that killed 34 people in December.
However, U.S. President Barack Obama has sought to reassure visitors, saying that while there are always some risks at large international gatherings, "Sochi is safe."