(Reuters) -United Airlines has asked a federal judge to dismiss a lawsuit claiming it unfairly charged passengers extra money to sit in "window seats" that, to their surprise, lacked windows.

Passengers filed proposed class actions in August against United and Delta Air Lines ( DAL ) after finding their seats on Boeing 737, Boeing 757 and Airbus A321 planes were windowless, which they say wasn't flagged during the booking process.

But in a Monday filing in San Francisco federal court, United said it never contractually promised that seats in the window position would have views, even when it used "window" to label those seats on seat selection screens and boarding passes.

"The word 'window' identifies the position of the seat-i.e., next to the wall of the main body of the aircraft," United said. "The use of the word 'window' in reference to a particular seat cannot reasonably be interpreted as a promise that the seat will have an exterior window view."

United, based in Chicago, also said courts have repeatedly held that federal law generally prohibits passengers from pursuing breach of contract claims concerning airline fees and surcharges, including the cost for more desirable seats.

Such fees, known as ancillary revenue, help carriers generate revenue while keeping base fares lower.

LAWYER FAULTS UNITED'S 'WORD GAMES'

Carter Greenbaum, a lawyer for the United plaintiffs, said in an email that United's position was "contrary to the reasonable expectations of countless passengers who unknowingly paid extra money for windowless window seats. Consumers deserve better than empty promises and United's word games."

Greenbaum also represents Delta passengers suing that carrier in Brooklyn, New York federal court.

According to the plaintiffs, passengers typically buy window seats to quell fear of flying and motion sickness, keep children occupied, get more light or watch the world as they fly past.

The plaintiffs said they would have chosen different seats, and not paid extra, had they known United and Delta would seat them next to blank walls.

Both lawsuits seek millions of dollars of damages, for more than 1 million passengers per carrier.