DALLAS >> The Federal Aviation Administration today warned Southwest Airlines and its mechanics union that their bitter fight could hurt the airline’s safety program.

The FAA’s top safety official sent a terse letter to the airline and the union saying that a breakdown in their relationship raises concern. He urged both sides to cooperate in complying with FAA safety standards.

However, the dispute appeared to escalate today. The leader of the Aircraft Mechanics Fraternal Association said his union filed a defamation lawsuit against Southwest and the airline’s chief operating officer.

It was the union’s response to Southwest, which sued AMFA last week in federal district court in Dallas over what it claims is an illegal work slowdown aimed at pressuring Southwest during bitter contract negotiations.

Southwest charges that mechanics are writing up minor maintenance items, leading to a surge in the number of planes taken out of service for repairs. It said write-ups include missing seat row numbers and other cosmetic flaws.

The airline has been canceling flights each day because the number of grounded planes has jumped from the normal 14 per day to as many as 62 one day in February, according to Southwest’s lawsuit.

Southwest will make its Hawaii debut on March 17.

This week, Southwest CEO Gary Kelly said the airline is losing “millions of dollars” a week because of the canceled flights.

Union officials charge that Southwest pressures mechanics to overlook safety issues in managers’ zeal to keep planes flying.

Southwest and the union for its 2,400 mechanics have been negotiating over a new contract for more than six years.

The union’s national director, Bret Oestreich, said the union does not want to fight in court, it only wants a new contract. He also urged his members to “continue to keep Southwest passengers and crew safe” despite “trying times.”

The FAA’s associate administrator for aviation safety, Ali Bahrami, said in his letter today that a breakdown in the relationship between Southwest and the union “raises concern about the ongoing effectiveness of the airline’s safety-management system.”

Despite the litigation, he continued, he urged both sides to work together to meet FAA safety standards. The letter did not allege any safety violations or investigations.

Bahrami’s letter was addressed to Oestreich and Southwest’s chief operating officer, Mike Van de Ven. It was first reported by The Wall Street Journal.

Government and industry insiders said the letter raising safety issues was unusual. Robert Mann, a longtime airline executive and consultant, said he could not recall a similar warning by the FAA in a labor dispute.

“Good for them for raising the issue,” he said. “Safety has to be paramount, superseding everything.”

However, Southwest spokesman Chris Mainz said it is not unusual for the FAA to emphasize safety “during such times at a carrier.”

“As always, we appreciate the FAA’s partnership and maintain our dedicated focus on assuring the highest level of compliance and safety,” he said.

The union declined to comment on the letter.

The FAA had already said it was increasing its oversight of Southwest, which an agency spokesman said at the time was standard during periods of labor-management tension.