CHICAGO (Reuters) - Southwest Airlines filed a lawsuit against its mechanics union on Thursday, accusing the workers of an illegal campaign to disrupt operations in order to improve their position in prolonged labor talks.

FILE PHOTO: Southwest Airlines planes are seen in front of the Las Vegas strip, Nevada, United States April 23, 2015. REUTERS/Lucy Nicholson/File Photo

Southwest and the Aircraft Mechanics Fraternal Association, which represents some 2,400 mechanics at the low-cost carrier, have been in contract negotiations since 2012.

In a lawsuit filed in Texas, Southwest asked a federal judge to order a halt to what it called an unlawful job action by the union.

AMFA did not immediately return a request for comment.

Tensions escalated when the union claimed on Feb. 11 that Southwest was not negotiating in good faith, according to the lawsuit.

Southwest said soon after it began experiencing “an uptick on cosmetic and other minor maintenance write-ups,” leading to an unprecedented number of aircraft out of service and forcing the airline to delay or cancel hundreds of flights.

For example, it said the number of write-ups for minor interior systems such as a missing row number on an airline that does not assign seats spiked almost 400 percent to 500 percent after talks broke down.

“It quickly became clear as these unusual write-ups spread across Southwest’s stations that something coordinated was beginning to occur,” the airline said in the lawsuit.

In addition to the cost and revenue loss for canceled flights, Southwest said experience showed that passengers whose flights were canceled could end up traveling on other airlines rather than re-booking with Southwest.

The union has disputed the notion that the maintenance issues were driven by the labor dispute, one of the biggest to hit a top-four U.S. airline in more than a decade.