US Transport Secretary Pete Buttigieg on Wednesday increased pressure on Southwest Airlines, saying that thousands of cancelled flights indicated a system failure at the low-cost carrier.
"We are past the point where they could say this is a weather-driven issue," Mr Buttigieg told ABC News.
"Don't get me wrong, all of this began with that severe storm. We saw winter weather affecting the country and severely disrupting all airlines."
Nationally, at least 60 people died in weather-related incidents in recent days, NBC News reported.
The rest of the aviation system and other airlines seemed to have recovered from the weather disruptions, Mr Buttigieg said.
"So what this indicates is a system failure and they need to make sure that these stranded passengers get to where they need to go and that they are provided adequate compensation, not just for the flights … but also things like hotels, like ground transportation, like meals, because this is the airlines' responsibility," he said.
Mr Buttigieg said he had spoken to the company's leadership.
US airlines cancelled thousands of flights as a severe winter storm swept over much of the country before and during the Christmas holiday weekend.
Southwest's woes, however, deepened while other airlines have largely recovered.
The airline has cancelled more than 14,500 flights since Friday.
On Wednesday it cancelled 2,500 flights by late morning, flight-tracking website FlightAware said.
Southwest Airline's chief commercial officer Ryan Green apologised for the chaos in a video statement on Wednesday night, and said the airline's preparation for the winter weather "just wasn't enough".
"We're continuing to work to make this up to you," he said, adding that customers will hear more on the airline's efforts to repair relationships soon.
Southwest told Reuters that it would reimburse customers for travel-related costs and had already processed thousands of requests by Tuesday.
Delta Air Lines said it had capped fares in all markets where Southwest operates, domestic and international.
The programme includes more than 700 non-stop markets and is valid for the rest of the year. United and American announced similar programmes.
Rick Larsen, the top Democrat on the House transport and infrastructure committee, tweeted that the company was treating flight cancellations as controllable beginning on December 24, which allows reimbursement for incidental expenses and refunds for full fares.
The airline's chief executive, Bob Jordan, said the low-cost operator needed to upgrade its legacy airline systems and apologised to customers and employees in a video message.
Shares of Southwest tumbled 6 per cent on Tuesday and were down about 1.5 per cent in morning trading on Wednesday.
Some analysts said the cancellations would affect profits in the fourth quarter.