Mechanics at Northwest Airlines Corp. walked off the job early today after negotiations with the carrier failed to reach a labor agreement by a midnight strike deadline.

The nationwide strike began at 12:01 a.m., according to the Aircraft Mechanics Fraternal Association, the union that represents Northwest's 4,400 mechanics, cleaners and custodians. The strike is the first major labor action against a large U.S. airline since 1998, when Northwest's pilots walked off the job for nearly three weeks.

"Northwest wanted a strike, and now they have one," O.V. Delle-Femine, the union's national director, said in a statement authorizing the walkout. "This will be difficult for our members and their families, too, but Northwest left us no choice."

In a statement, Northwest said its final offer is "fair to our employees" while meeting the airline's need to reduce annual labor costs by $1.1 billion. The carrier said it planned to operate a normal schedule of domestic and international flights despite the strike.

"Northwest customers can continue to depend on Northwest for their travel needs," Douglas M. Steenland, the airline's president and chief executive, said in a statement.

Disruptions are expected to be felt most deeply in the Midwest -- especially in Detroit and Minneapolis -- where Northwest operates the majority of its flights. The airline also has a significant overseas presence, with service to Asia and Europe through hubs in Tokyo and Amsterdam. Northwest said travelers can check the status of their flights on the airline's Web site, www.nwa.com, or on its flight information line at 800-441-1818.

Officials at the Washington area's three major airports said they did not expect significant travel disturbances or major demonstrations, owing to Northwest's relatively small presence here. The airline accounts for 4.8 percent of the flights at Reagan National Airport, 4.25 percent at Baltimore-Washington International Airport and 1 percent at Washington Dulles International Airport.

Northwest, the nation's fourth-largest carrier, has said it has lost more than $3.6 billion since 2001 and is on the brink of filing for Chapter 11 bankruptcy protection. The airline had sought $176 million in concessions from the mechanics. The union said the airline wanted to slice the mechanics workforce in half by using more outside contractors and to cut the pay of remaining workers by 25 percent.

Representatives from Northwest and the union negotiated through the day and evening yesterday in a final effort to reach an agreement. In talks at the National Mediation Board headquarters, both sides debated what the airline called its "last best offer," presented to the union late Thursday evening. The union described that proposal as unacceptable.

In preparation for a walkout, Northwest had lined up 1,300 replacement mechanics willing to work for lower wages, and it had said a strike would not disrupt operations. Most of the replacements have been laid off from other carriers. The rest of the work would be outsourced, the airline said.

Northwest's flight attendants, ground workers and pilots said they would not join the strike, the Associated Press reported.

The mechanics union offered little hope earlier yesterday of avoiding a walkout. Its members spent the day setting up trailers and staking out areas for picket lines at various airports.

Tara Hamilton, a spokeswoman for National and Dulles airports, said the Metropolitan Washington Airports Authority has issued permits that would allow up to 40 picketers at a time at National and up to 12 at Dulles.

At National, demonstrators would be permitted at the curb outside Terminal A, while at Dulles, they would be allowed inside the terminal near the Northwest ticket counter. Hamilton said Northwest would provide extra security, if needed, to ensure that passengers and operations would not be disrupted.

BWI spokeswoman Cheryl Stewart said the airport has not issued any protest permits. But union spokesman Steve MacFarlane said the mechanics do expect to mount a picket line at BWI.

The Federal Aviation Administration said it is ready to deploy 65 safety inspectors throughout Northwest's hub-and-spoke system as the carrier's mechanics are replaced with temporary workers. "We'll be watching any strike transition every step of the way to make sure Northwest continues to operate safely," FAA spokesman Greg Martin said.

Staff writer Sara Kehaulani Goo contributed to this report.