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.”