Rivals quickly match the price cuts by United, which struggled through labor woes and poor performance this summer.
United Airlines has reduced fall fares substantially in an effort to win back business and shore up its image after a summer of labor strife, delays and cancellations. Other carriers matched the lower fares.
The world's largest airline said fares on many of its domestic round-trip flights through Dec. 14 will be reduced by about half, while longer U.S. and international flights will be cut back less sharply.
At least seven other U.S. carriers had matched the sale by Wednesday morning, although some had more restrictions than United, which announced its fare cuts Tuesday.
US Airways matched fares on competing routes with United out of Tampa International Airport. Both carriers offer round-trip fares of $248 to Chicago's O'Hare International Airport, $298 to Denver and $498 to Los Angeles. Tickets must be purchased by Sept. 22 for travel by Dec. 14. Prices do not include fuel surcharges.
The move comes less than a week after the nation's major airlines raised domestic fares by adding a $20 surcharge to cover the higher cost of jet fuel.
Sample round-trip fares include Boston-Denver for $288, Chicago-New York for $198, Los Angeles-New York for $598, Washington, D.C.-Orlando for $198, New York-London for $228, Denver-Munich for $448 and Chicago-Frankfurt for $508.
Trying to lure more business to its Web site, United also said additional discounts will be given to those who book online at www.united.com.
Discounts are available on tickets purchased through Sept. 22. United requires a seven-day advance purchase. There are no blackout dates, and no fuel surcharge applies.
Airline expert Joe Brancatelli, who writes a column for BizTravel.com, said the move reflects United's desperation at the extent of this year's problems. He said the airline gains no real advantage by launching a general fare sale since others matched it.
"They must have such a gigantic shortfall in traffic, they had no other choice," he said.
"In August, with a 12 percent traffic decline, September and October and the whole fall must look disastrous" for the airline, he said. "So they had to do something drastic."
Travel analyst Terry Trippler of onetravel.com said the United move was matched in some form by American Airlines, America West, Continental Airlines, Delta Air Lines, Northwest Airlines, TWA and US Airways.
_ Times staff writer Steve Huettel contributed to this report.