The Homeland Security Department has alerted air carriers to a potential terrorist tactic involving insulated beverage containers such as thermoses.
The alert stressed that there is no active intelligence about a terrorist plot but that travelers may notice airport screeners taking a closer look at empty insulated containers.
The Transportation Security Administration "is carefully monitoring information related to terrorist tactics" in coordination with other nations, TSA spokeswoman Sterling Payne said in a statement Thursday.
"The possible tactics terrorists might use include the concealment of explosives inside insulated beverage containers, so in the coming days, passengers flying within and to the U.S. may notice additional security measures related to insulated beverage containers," she said.
Payne urged the public "to remain vigilant and aware of their surroundings and report any suspicious activity to their local authorities."
Homeland Security regularly alerts law enforcement about evolving terrorist tactics as counterterrorism officials learn about them through intelligence chatter.
Meanwhile, New York City is "on alert" for possible terrorist attacks during the holiday season, although there is no information about specific threats to the largest U.S. city, Police Commissioner Raymond W. Kelly said Thursday.
The Police Department is taking additional steps to protect the city, such as deploying about 1,200 recruits graduating next week to the area around Times Square for New Year's Eve, Kelly told reporters after a promotion ceremony at Police Headquarters in Manhattan.
"We have no specific information, but we're on alert," Kelly said. "We're watching indicators that we normally watch. We're watching them closely, but there's no indication of a specific threat to this city at this time."