The U.S. Department of Transportation fined American Eagle Airlines $900,000 for keeping 600 passengers waiting on the runway for more than three hours in 15 of its airliners as a result of flight delays at Chicago's O'Hare International Airport.

This was the first offense against the rule imposed by the federal government in April 2010 for tarmac delays of domestic flights.

"We put the tarmac rule in place to protect passengers, and we take any violation very seriously," Transportation Secretary Ray LaHood said in a statement. "We will work to ensure that airlines and airports coordinate their resources and plans to avoid keeping passengers delayed on the tarmac."

The fine stemmed from incidents that occurred on May 29 when American Eagle, a subsidiary of American Airlines, kept 15 flights grounded on the O'Hare runway for more than three hours.

A total of 608 passengers had to stay more than three hours inside the planes on the landing strip with no way to get back into the terminal, according to a Department of Transportation investigation.

Of the total sanction, $650,000 must be paid within 30 days, while the remaining $250,000 will go to the affected passengers either in the form of travel mileage, coupons for other flights or cash reimbursements.

"We take our responsibility to comply with all of the (Tranportation) department's requirements very seriously and have already put in place processes to avoid such an occurrence in the future," he said.

The regulation obliges companies to return airliners to the terminal when the three-hour limit is up, or offer an alternative way for passengers to leave the plane.

Shortly afterwards the government extended the regulation to include international flights in U.S. airports, though it increased the permitted waiting time on the runway to four hours.