DALLAS (CBSDFW.COM/AP) — A spike in Southwest Airlines planes ruled out of service for mechanical issues is costing the carrier millions each week because of delayed and canceled flights, the CEO says.

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 lawsuit accuses the union of encouraging mechanics to write up issues that Southwest said has no effect on the safety of flights in order to gain leverage on in ongoing contract negotiations.

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

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.

The FAA has stepped up oversight of Southwest’s maintenance, which is standard when an airline is experience labor issues.

(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)