Passengers on nonstop flights between the United States and Russia have been prohibited from packing liquids, gels, aerosols or powders in their carry-on bags during the Winter Olympic Games in Sochi under a new directive issued by the United States Transportation Security Administration.

The ban, which Delta Air Lines announced on its website on Thursday, was imposed amid concerns that terrorists might try to assemble explosives in the air or at the Games from materials smuggled onto planes. The Department of Homeland Security warned airlines about that possibility on Wednesday, although the agency added that it was not aware of a specific threat.

“Our security posture, which at all times includes a number of measures both seen and unseen, will continue to respond and appropriately adapt to protect the American people from an ever-evolving threat picture,” a department official said. “These measures include intelligence gathering and analysis, deployment of cutting-edge technology, random canine team searches at airports, federal air marshals, federal flight deck officers, temporarily restricting certain items and more security measures both visible and invisible to the public.”

The temporary restrictions exclude prescription medications and do not apply to checked bags.

Delta Air Lines, the only American carrier with regularly scheduled direct flights to Russia, said passengers traveling to Russia must check in with an airline representative, instead of online or at electronic kiosks.