American Eagle was hit with a $900,000 fine for keeping hundreds of passengers cooped up on planes in Chicago earlier this year, the Department of Transportation said Monday.

The American Airlines affiliate encountered delays of over three hours on 15 flights arriving at Chicago O'Hare International Airport on May 29, inconveniencing 608 travelers, according to DOT officials.

The hefty fine was a warning to airlines that extreme delays will no longer be tolerated as we approach the busiest time of year for air travel.

The airline blamed the delays on airport congestion due to a slow-moving weather system, adding that it has apologized and provided travel vouchers or frequent flyer mileage credit to affected customers.

We take our responsibility to comply with all of the department's requirements very seriously and have already put in place processes to avoid such an occurrence in the future, American Eagle President and CEO Dan Garton said in a statement.

In April of last year, the DOT introduced new regulations that require a domestic flight to return to the gate or enable passengers to exit the aircraft if tarmac delays exceed three hours. The same rule applies after four hours for international flights.

According to the new rule, airlines in violation can be fined up to $27,500 per passenger.

The steep fine issued to American Eagle marks the first time the penalty has been enforced since the rule was implemented last year. While much larger fines have been issued to airlines for incidents that violated federal safety regulations, this is the largest fine paid by a carrier in a consumer protection case not involving civil rights violations.

American Eagle has 30 days to pay $650,000 of the fine. The remaining $250,000 can be used to compensate passengers on the 15 affected flights, as well as future disrupted flights, the DOT said.

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, he added.

The 2010 rule came into effect after a series of incidents in which passengers complained of being kept against their will on planes in sight of an airport terminal.

American Eagle isn't alone in forcing angry passengers to remain inside a plane on the tarmac. On Oct. 29, three JetBlue planes and one American Airlines plane sat on the tarmac at Bradley Airport in Connecticut for over seven hours as an early Nor'easter hit the region. The passengers were eventually let off the planes, but had to spend the night on cots and chairs in the airport terminal.