NEW YORK (AP) _ Six more major U.S. airlines have announced Christmas holiday discounts of up to 80 percent on most domestic routes, but the fares are $40 more expensive than the industry's Thanksgiving sales.

American Airlines, United Airlines and Western Airlines said Tuesday they will offer special fares from Christmas Day until midnight Dec. 27, for non- refundable, roundtrip tickets purchased by Dec. 23.

Trans World Airlines and Republic Airlines matched the offer and extended it by one day until midnight Dec. 28.

Continental Airlines announced it would match the special fares on routes where its regular fare was higher than the special Christmas fares. Spokesman Michael Cinelli contended Continental's regular fares already were lower or equal to the special fares offered by the others.

Delta, Eastern, People Express and Piedmont announced earlier that they would apply discounts to the Christmas period.

Republic spokesman Robert Gibbons said its sale was ''in reaction to what the other airlines proposed.''

TWA spokeswoman Sally McElwreath said its sale also applies to the New Year period, Jan. 1-3, but passengers traveling then must buy tickets by Dec. 30. She also said Christmas and New Year flights cannot be combined.

One-way prices will be $49 for routes up to 500 miles in length, $69 for 501-1,500-mile routes and $99 for routes exceeding 1,500 miles, the airlines said.

American was first to offer the Thanksgiving sale, which featured discounts of up to 85 percent for 21/2 days, or $29 each way for trips of 500 miles or less, $49 for 501-1,500 mile trips and $79 for trips exceeding 1,500 miles.

Asked why the discount was reduced for the Christmas holiday, Michael W. Gunn, senior vice president for passenger marketing, said in a telephone news conference: ''With the three full days of travel, we feel it's logical to move the price up slightly.''

United spokesman Charles Novak said in a telephone query that United originally had not intended to offer a Christmas sale, but reversed the decision because other carriers were discounting holiday tickets.

The sales are designed to lure impulse travelers who otherwise would not fly over the holidays, when traffic usually is light. The airlines are hoping to fill hundreds of thousands of empty seats, making up in volume what they lose in cheaper tickets.

Several carriers reported last week that the Thanksgiving sale did not boost ridership as much as they had hoped, but still described the discounting as a success.