Northwest Airlines Corp., the fifth-largest U.S. carrier, canceled at least 240 weekend flights because it didn't have enough crews, the pilots union said.

"We have been saying for months that there is a staffing problem at Northwest," Monty Montgomery, spokesman for the Air Line Pilots Association, said Monday in an interview. "There are just not enough pilots to fly the schedule as it's designed."

Northwest had crew shortages due to bad weather and is "proactively canceling flights" to speed rebookings, the Eagan, Minn.-based carrier said in an e-mailed statement. Spokesman Dean Breest wouldn't say how many were canceled.

The cancellations are adding to U.S. travel disruptions including thunderstorms, crowded planes and congested airspace. U.S. airlines managed only 72.5 percent of flights on time this year through April, the worst rate since the federal government began keeping track in the current format in 1995.

The flights cited by the pilots represent about 5 percent of Northwest's weekend flying, based on totals from schedule publisher Official Airline Guide. Northwest said it's advising customers to check the status of their flights online at www.nwa.com or by telephone at 1-800-441-1818.

Storms can create crew shortages by stranding pilots far from where they're supposed to fly, or by lengthening their duty hours until they lack enough hours under federally mandated schedule limits to complete a flight.

Most other U.S. carriers reported no recent weather- or crew-related cancellations.

Terry Trippler, who monitors travel and fares for myvacationpassport.com, said the Northwest cancellations may be related to pilots' unhappiness with airline executives, including a June 15 union resolution declaring a lack of confidence in management. The union also has staged protests against stock and options awarded to Chief Executive Officer Doug Steenland when Northwest exited bankruptcy on May 31.

"We've got a bit of a slowdown going on here, and their message is coming through loud and clear," Trippler said.

Montgomery denied the weekend cancellations were related to disgruntled pilots trying to slow the airline's performance. "Our pilots were tired already and now the summer months are coming and it's beginning to take its toll," he said.

In the no-confidence resolution, the Air Line Pilots Association wrote that a "staffing shortage will have a noticeable and costly impact on NWA's summer flying, revenue and passenger goodwill."

Northwest also canceled an unspecified number of flights in October because of a pilot shortage, the union said. Union representatives and Northwest managers met at that time to discuss the cancellations.

AMR Corp.'s American Airlines, UAL Corp.'s United Airlines, Delta Air Lines Inc., Continental Airlines Inc., Southwest Airlines Co. and JetBlue Airways Corp. said they have had no recent weather- or crew-related cancellations.

Recent weather delays forced US Airways Group Inc. to cancel some flights as crews reached the federally mandated limit on duty times, spokeswoman Andrea Rader said in an interview. "It's maybe a slight bump over what you'd see on a typical day for the summer," she said.

United on Wednesday halted all takeoffs for more than two hours after the computer that controls flight operations failed.