Amazon’s holiday season has become far more complicated.

Approximately 250 pilots who fly cargo carriers for ABX Air went on strike Tuesday morning and have declined to fly any of their scheduled routes until they can come to an agreement with their employer. ABX, which is owned by the Air Transport Services Group (ATSG), has several cargo customers, but its two biggest are Amazon and DHL. And now the pilots and the Airline Professionals Association, Teamsters Local 1224 union supporting them are saying Amazon’s shipments could be impacted, leading to delivery delays for the e-retail giant’s customers.

According to the pilots, the cargo carrier is flying 14 aircraft and 35 flights per day for Amazon AMZN , carrying the company’s goods from one spot to another to facilitate shipments to customers. In a statement, the pilots say “Amazon customers will also see delays and disruptions.” DHL relies on Air Transport Services Group for 45 flights per day.

The row between the pilots and ABX is due in part to what the pilots say is a company that is “significantly understaffed.” They claim ABX Air and ATSG are violating their employment contract by forcing them to work “emergency assignments” when they’re not scheduled to fly, and not allowing them to take extra time off for their additional work.

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“I take my job as a pilot seriously, and I’m committed to serving ABX Air and our customers, but I’m also a father of a little girl and help care for my aging mother,” Randy Riesbeck, an ABX pilot, said in a statement. “On numerous occasions I have had to miss my daughter’s school events and previously scheduled medical appointments for my mother, all because ABX Air emergency assigned me to work on a day I had scheduled off. How am I supposed to explain to my daughter why I wasn’t there to see her grow up? How do I explain to my mother that I can’t take her to the doctor?”

The pilots add that their work schedule is “causing stress and strain” and say their employers might be violating a U.S. law governing work and compensation.

“To date in 2016, pilots have been scheduled to cover over 8000 emergency assignment days on days they should have had off,” a statement reads. “As the airline moves into peak holiday flying season, ABX Air and ATSG are refusing to honor contractual provisions regarding compensatory time and vacation time in order to cover the pilot deficit. This change violates the terms of the company’s agreement with the pilots, which is a violation of the status quo under the Railway Labor Act.”

ABX Air said in a statement on Tuesday that it believes the strike is “illegal” and says it will use the court system to put an end to the stoppage.

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“We will seek a court order later today to restore the status quo operating environment, even as we continue discussions with union representatives about specific issues of concern,” ABX Air President John Starkovich said in a statement. “We expect the court will uphold our position that the actions taken by the union to refuse work assignments is not legal, and the issues involved constitute a minor dispute to be resolved via arbitration under terms of our current labor agreements.”

The chief executive added that its customers might face “temporary interruptions” to their service, but didn’t say how far-reaching the delays might be.

An Amazon spokesperson didn’t say how the work stoppage might affect the company’s shipments and customer service.

“We work with a variety of carriers and are confident in our ability to serve customers,” the spokesperson said in a statement. It’s unclear how much of Amazon’s logistics network relies upon the 35 affected flights.

In a statement to Fortune, an ABX Air spokesperson did not elaborate beyond Starkovich’s public comments.

The strike comes just a day after workers at Chicago O’Hare International Airport, one of the busiest airports in the world, announced a strike ahead of the busy holiday travel season.