DALLAS (AP) — The CEO of Southwest Airlines says a spike in planes ruled out of service for mechanical items is costing the carrier millions each week because of delayed and canceled flights.

Gary Kelly didn't give a precise figure Tuesday, but said the financial damage prompted the airline's lawsuit last week against the Aircraft Mechanics Fraternal Association.

"The company filed suit against AMFA last week to recover those damages and prevent more from occurring," Kelly said at a JPMorgan investor conference.

Southwest says some workers are writing up minor mechanical problems and grounding planes to gain leverage in negotiations for a new labor contract.

The union didn't comment immediately. Last week, AMFA national director Bret Oestreich said Southwest produced no evidence that any mechanical write-ups were improper.