The U.S. Department of Transportation said Friday it fined Frontier Airlines US$1.5 million for keeping passengers stuck on a dozen aircraft on the Denver airport tarmac for more than three hours amid a snowstorm last December.

But the department said it will forgive US$900,000 of that because of compensation the airline paid to the passengers.

The delays came after a storm dropped 8 inches of snow at Denver International Airport on Dec. 16-17.

Passengers were held aboard 12 planes sitting on the tarmac for more than the three-hour limit set by law, the Transportation Department said.

The department said Frontier did not have enough staff and did not delay or divert enough flights to alleviate congestion at airport gates.

On one occasion, the airline used an open gate to load passengers on a departing flight instead of unloading a waiting plane that had already arrived, the department said.

Frontier spokesman Richard Oliver said the airline revised its winter weather procedures and worked with the Denver airport on a system to get passengers off planes more quickly.

"During last December's crippling storm, our operation in Denver was faced with a myriad of operational challenges," he said in a written statement.

The airline told the Transportation Department that the snowstorm was worse than predicted and that it had taken steps to relieve the delays. The airline said it was fully staffed and called in extra workers from its Denver headquarters to help at the airport.

Frontier told the department it spent nearly US$1.2 million on compensation and flight vouchers to passengers.

The Transportation Department said the US$1.5 million fine was the second-highest amount imposed on an airline for tarmac delays. Southwest was fined US$1.6 million in 2015 and United the same amount in 2016.