Northwest Offers Deep Fare Cuts : Air wars: Renewed price-cutting in peak travel season means competitors must follow suit or risk losing customers.

In a new round of summer fare cuts, Northwest Airlines on Thursday cut its discount fares by up to 33% for travel in North America and on selective routes to Europe.

The sale, which expires May 18 and applies to a limited number of seats, is one of the most far-reaching this summer. Most airlines competing with Northwest--the nation’s fourth-largest carrier--will be compelled to match the promotion or risk losing customers.

In the Northwest sale, the non-refundable tickets must be purchased two weeks before departure and require a Saturday-night stay-over. The discounted fares apply to trips taken from May 27 through Sept. 15. For flights to and from Canada, the discount fares apply to trips taken between June 6 and Sept. 15.

Trips to be completed by June 30 qualify for the deepest discounts--as much as 33% off. Afterward, discounts will range up to 30%.

Northwest and American Airlines last summer had 50%-off sales, which many travelers hoped would be repeated, but Northwest said its advance summer bookings were meeting expectations and indicated no desire to resume such deep discounting.

“Airline fares--even sale fares--have been trending up, and we expect this to continue,� Northwest marketing executive Michael E. Levine said. “For customers who have been waiting to book summer travel, this is the best opportunity to plan vacations.�