WASHINGTON (Reuters) - Boeing ( BA ) said late on Thursday it filed an unfair labor practice charge with the National Labor Relations Board against the union that represents about 33,000 striking U.S. West Coast factory workers.

The planemaker said the International Association of Machinists and Aerospace Workers unit representing the workers had failed to bargain in good faith during the four-week work stoppage.

Boeing ( BA ) said the union had engaged in a "pattern of bad faith bargaining, and of issuing misinformation to its members about the status of negotiations."