American Airlines was fined $1 million and Delta Air Lines Inc. $750,000 for violating federal rules barring lengthy tarmac delays at U.S. airports, the U.S. Transportation Department said on Thursday.

Under most circumstances, airlines are prohibited from allowing domestic flights to remain on the tarmac for longer than three hours, and international flights for more than four hours, without giving passengers an opportunity to deplane.

Of the fine assessed to American, $450,000 was credited to the airline for compensating passengers. The Transportation Department cited 13 tarmac delays since December 2015 at American Airlines and blamed "gate mismanagement" for two delayed flights.

Delta was credited with $450,000 for compensating customers and establishing a backup data center and an automated aircraft-parking guidance and jet-bridge positioning system, the department said.

The airlines agreed to pay the resulting fines as part of consent orders.

Delta said it provided customers with substantial compensation for the delays, including cash reimbursements, SkyMiles and future travel vouchers.

"Delta has spent millions to invest in new technologies to increase efficiency of aircraft movement during irregular operations and additional deicing capacity at its Hartsfield-Jackson Atlanta International Airport hub," the company added.

American Airlines spokeswoman Shannon Gilson said the delays were the result of "exceptional weather events dating as far back as 2015." The company disputed that gate mismanagement was to blame for two delays.

She said the airline had "put procedures in place to help better accommodate our customers when inclement weather occurs."

American Airlines told the department it has invested more than $5 million in additional capital investments like vans, air stairs and lavatory trucks to address tarmac delays.

The Transportation Department cited about a dozen tarmac delays at Delta since 2017 and said passengers on seven flights at Atlanta were not allowed to deplane for hours even though there was "no safety, security or Air Traffic Control reason for not doing so."

Delta said the "delays were the direct result of an extraordinary, unanticipated, and unpredictable system outage in the data center at Delta's Atlanta hub."

Delta has invested more than $250 million to set up a new data center to back up essential applications in case of problems at the primary data center, the company said.

Delta has also spent $12.4 million on an automated aircraft-parking guidance and jet-bridge positioning system and is purchasing 20 additional deicing trucks and planning additional deicing pads at Atlanta.