DALLAS (AP) — A high-ranking federal regulator is telling Southwest Airlines and its mechanics union that their bitter fight could hurt the airline’s safety program.

The Federal Aviation Administration’s top safety official sent a terse letter Friday, saying both sides have a safety responsibility.

Ali Bahrami says a breakdown in the relationship between the airline and the union raises concern about the effectiveness of Southwest’s safety-management system. He’s urging both sides to cooperate.

The dispute appears to be escalating, however. The top official at the union, the Aircraft Mechanics Fraternal Association, says the union filed a defamation lawsuit against Southwest.

Last week, the airline sued the union and six representatives over what it claims is an illegal work slowdown aimed at pressuring Southwest during contract negotiations.