CHICAGO (Reuters) – American Airlines Group Inc has filed a lawsuit against two unions representing its mechanics, asking the court to halt what it called an illegal slowdown that it said on Monday threatened to disrupt U.S. summer travel.

The lawsuit, filed in the Northern District of Texas federal court, accuses the mechanics unions of directing a slowdown aimed at disrupting operations to improve their position in labour talks, which began in 2015.

The unions did not immediately return a request for comment.

American, the world’s largest airline by passenger traffic, said the mechanics’ action had intensified in recent weeks after causing 650 flight cancellations and more than 1,500 maintenance delays since February.

“American estimates that, for each day that the slowdown continues into the summer, it will disrupt the travel plans of approximately 3,400 additional passengers per day,” the airline said in the filing.

American said it has offered the Transport Workers Union of America and the International Association of Machinists an industry-leading proposal in all key respects, including pay and benefits.

The unions have complained that American is trying to outsource more maintenance jobs, a move American has indicated is necessary to cover increased wages.

Fort Worth, Texas-based American last year asked the National Mediation Board to help negotiate an agreement with the mechanics unions, but it said discussions were paused after the last session on April 25. Since then, it said union action had caused 14 flight cancellations per day, versus an average eight daily union-related cancellations previously.

The airline is also in contract talks with unions representing its flight attendants and pilots, whose contracts become amenable in 2019 and 2020 respectively.

Low-cost rival Southwest Airlines Co recently reached an agreement in principle with its mechanics union after the Federal Aviation Administration warned that their prolonged contract dispute could pose safety concerns.

Shares in American closed 2.46 percent lower on Monday at $30.96.