TWA, the 8th-largest U.S. airline, took the lead by posting a sale Monday and was quickly followed by Continental Airlines , the 5th-largest carrier. United Airlines, the world's biggest airline, and US Airways, the No. 6 U.S. carrier, weighed in Tuesday.

Northwest Airlines Corp., the No. 4 U.S. airline, told Reuters it would also lower fares. A spokesman for AMR Corp.'s American Airlines, the No. 2 world airline, told Reuters it would match United's fare sales.

Michael Stellwag, an airline analyst at Warburg Dillon Read, said a fare sale at this time was not unusual.

"Between New Year's and spring break, you really get a lull in passenger traffic," he said.

United is lowering fares by up to 30 percent below standard 21-day advance-purchase fares for both domestic and international flights, a spokesman told Reuters.

Representatives of TWA, which began the round of fare cuts, were not immediately able to give a range of the cuts. But an industry source, who asked not to be identified, said TWA's cuts ranged up to 42 percent, meaning United appeared not to match the full range of cuts by the much smaller TWA.

Delta Air Lines Inc., the No. 3 U.S. carrier, was not immediately available to comment.

The airlines lowered fares for travel from roughly mid-January to mid-May, requiring purchase by Jan. 14 and at least 14 days ahead of the departure date.

The greatest price reductions are available for travel through roughly mid-January. The United and US Airways sales also extend for travel through May.

UAL Corp.'s United added an incentive for customers to book tickets on its Web site (http:/www.ual.com), offering up to 5 percent off and 1,000 bonus miles. First-time online customers will receive an additional 3,000 bonus miles, showing how carriers are ratcheting up their strategies for selling tickets online.

US Airways also offered 5 percent off its own domestic and international sale prices for buyers using its Web site (http:/www.usairways.com).