(Reuters) - A top Southwest Airlines Co executive said the low-cost carrier would investigate why the number of aircraft unable to fly due to mechanical issues had doubled in recent days, causing flight delays and cancellations.

FILE PHOTO: A 737 MAX 8 produced for Southwest Airlines is pictured in Renton, Washington, U.S. March 13, 2018. REUTERS/Jason Redmond/File Photo

“AMFA has a history of work disruptions, and Southwest has two pending lawsuits against the union,” Chief Operating Officer Mike Van de Ven said in a statement. Aircraft Mechanics Fraternal Association (AMFA) represents about 2,400 mechanics who maintain the carrier’s Boeing 737 fleet.

Southwest has been in negotiations with the union representing its mechanics for six years.

Van de Ven said the disruption had arisen despite no change to the low-cost carrier’s maintenance programs, leadership, policies or procedures.

“We will be investigating this current disruption and exploring all possible remedies,” Van de Ven said.

Flight cancellations by Southwest accounted for roughly 24 percent of nearly 800 total flights canceled across the United States on Tuesday, according to flight-tracking service FlightAware.com.

Southwest said about half of its cancellations had been related to unscheduled maintenance issues, and the remainder due to winter storms forecast to hit the Midwest and Northeast over Tuesday and Wednesday.

Last week, Southwest said it was requiring “all hands on deck” to address maintenance items and said all employees would have to provide a doctor’s note if they failed to show up for work.

In response, the union said Southwest’s requirements imposed extra-contractual demands with a threat of termination for failing to comply, and asked for its contract to be respected.

“We are only asking that we be permitted to perform our job in accordance with federal law – nothing more and nothing less,” the union said in a memo posted on its website.