MINNEAPOLIS (AP) _ The machinists' union at Northwest Airlines has asked the National Mediation Board to intervene in its long-standing contract dispute with the airline, according to a union official who says the request means the union won't strike Sunday.

District Lodge 143 of the International Association of Machinists (IAM) filed the request Tuesday, and John Massetti, the union's secretary-treasu rer, said he expected the board would assign a mediator.

The airline declared an impasse in the on-again-off-again contract talks last Wednesday. That step started a 10-day waiting period after which the union could strike or the company could impose its last contract offer.

The IAM said Tuesday it asked for mediation to avert a strike and prevent Northwest from imposing new contract terms on the 2,700 mechanics, who worked for Republic Airlines before it was acquired by Northwest last year.

Northwest and IAM negotiators have been trying for months to negotiate a new contract that would eliminate wage and fringe-benefit disparities and bring former Republic and former Northwest ground employees under common contracts.

The IAM's 20,000 members work under four contracts, so there was some dispute over how many could strike. Northwest contends the talks cover only the 2,700 former Republic mechanics, whose contract became amendable in 1986. The IAM claimed that as many as 10,000 members - the number who do not have a post-merger transition contract - could have gone on strike.

Terry Erskine, Northwest's vice president for law and labor relations, said late Tuesday he would not comment until he sees the IAM petition. He said Northwest would be willing to resume contract talks if the government furnishes a mediator.