A Southwest Airlines Boeing 737-700 plane sits on the runway waiting to take off at LAX airport in Los Angeles, California U.S. January 10, 2018. REUTERS/Lucy Nicholson

(Reuters) - The Federal Aviation Administration (FAA) warned Southwest Airlines Co and its mechanics union that their ongoing labor dispute threatens to damage the airline’s safety practices, the Wall Street Journal reported on Friday.

Southwest and the Aircraft Mechanics Fraternal Association, which has been in contract negotiations since 2012, are locked in an escalating battle.

Last week, the airline filed a lawsuit against the union accusing the workers of an illegal campaign to disrupt operations in order to improve their position in prolonged labor talks.

The FAA’s top safety official, Ali Bahrami, issued a letter to the airline that said conflict between the low-cost carrier and the union “raises concern about the ongoing effectiveness of the airline’s safety management system,” the WSJ reported.

The union represents some 2,400 of Southwest’s mechanics.

Southwest, the mechanics union and the FAA did not immediately respond to a Reuters request for comment.