WASHINGTON — The Obama administration took aim today at tarmac horror stories, ordering airlines to let passengers stuck in stranded airplanes get off the plane after three hours.

With its new regulations, the Transportation Department sent an unequivocal message on the eve of the busy holiday travel season: Don't hold travelers hostage to delayed flights.

The Air Transport Association, a trade group that represents U.S. airlines, said carriers would comply with the new rule.

Under the new regulations, airlines operating domestic flights will be able only to keep passengers on board for three hours before they must be allowed to disembark a delayed flight. The regulation provides exceptions only for safety or security or if air traffic control advises the pilot in command that returning to the terminal would disrupt airport operations.

U.S. carriers operating international flights departing from or arriving in the United States must specify, in advance, their own time limits for deplaning passengers. Foreign carriers are not covered by the rules.

Airlines will be required to provide food and water for passengers within two hours of a plane being delayed on a tarmac and to maintain operable lavatories. They also must provide passengers with medical attention when necessary.