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Two major U.S. airlines were ordered to pay large fines Thursday for keeping passengers waiting in planes on tarmacs for more hours than allowed under federal rules.

American Airlines was fined $1 million after the Department of Transportation found that between December 2015 and January 2017, 10 of its domestic flights and three international flights remained on the tarmac at various U.S. airports beyond the time allowed, the DOT said on its website.

Passengers cannot be kept on a tarmac on domestic flights for more than three hours without being given the opportunity to deplane, according to the federal rule. Travelers on international flights cannot be kept on their flight on a tarmac for longer than four hours.

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American Airlines violated the rule twice in 2015, nine times in 2016 and twice in 2017.

The latest incidents happened in January 2017 on two flights that were diverted to airports other than where they were headed. In both cases, American "failed to offer passengers the opportunity to deplane," the DOT said.

American told the agency in a statement included in the DOT report that "it takes very seriously its responsibility to comply with all of the Department’s requirements" but also tries to get travelers to their intended destination rather than deplane them at the diverted location.

Delta Air Lines was fined $750,000 after 11 flights were left on the tarmac for longer than allowed between January 2017 and February 2018, according to the federal agency.

Seven flights were left on the tarmac for longer than three hours in January 2017 due to a systems outage at an Atlanta airport. In December of that year, one Delta flight was left on the tarmac in Atlanta for nearly four hours.

Passengers on three separate flights in February 2018 weren't allowed off their planes until at least three hours after arriving at the gate.

Both airlines will be credited $450,000 toward their fines for "compensation provided to passengers," the DOT said.