Delta Air Lines Inc., Continental Airlines Inc., United Airlines and US Airways Group Inc. have each cut flights to Mexico following a reduction in demand due to the swine flu outbreak.

The service reductions to Mexico, where the flu formally known as H1N1 originated, are the first by U.S. carriers. American Airlines is the only one of the five biggest U.S. carriers not to have reduced their number of flights.

New cases of the flu, which has spread to 12 countries, may further affect international travel.

Continental, with the most flights to Mexico of any airline at 450 a week, cut that seating capacity in half and put smaller jets on some routes to keep providing service to all 29 of its Mexico destinations.

The new schedule begins May 4, the Houston- based company said yesterday.

Delta said it will trim some of its 350 weekly flights to Mexico while preserving service to all its 11 destinations in the country.

United Airlines parent UAL Corp. will slash weekly flights to Mexico to 24 from 60 for May and to 52 from 90 during June.

US Airways is trimming flights by 38 percent, also by switching to smaller aircraft and reducing frequency. Schedules will be re-evaluated in two weeks to determine whether further adjustments would be needed.