New York (AFP) - More than 10,000 flights cancelled over the Christmas holiday, chaos at airports across America: Southwest Airlines found itself in the hot seat Tuesday as the airline behind the lion's share of the weather-linked travel mayhem.

The Dallas-based, domestic-focused carrier, which has historically enjoyed a strong reputation with consumers, drew withering explicative-filled rebukes on social media, where labor leaders also highlighted horror stories from stranded airline employees.

"It's a complete meltdown," said Mike Sage, who had planned to fly back Monday to Florida after visiting Connecticut to tend to his kayaking business.

After Southwest's phone and Internet system "collapsed," Sage drove to the airport, waited in line for two hours and finally obtained a replacement ticket for Saturday from Southwest.

But it came with a warning, Sage recounted to AFP.

"When (the attendant) handed me the ticket, she looked me in the eyes and said: 'If I were you, I would not count on this flight either. I would book with another airline. We have crews stranded all over, pilots sleeping on the floor in airports.'"

At issue is Southwest's performance in the wake of a brutal winter storm that began ahead of Christmas, wreaking havoc with holiday travel networks and causing some 50 fatalities.

But while operations had largely returned to normal at American Airlines and United Airlines by Tuesday, Southwest canceled more than 2,500 flights, or nearly two-thirds of planned departures, according to tracking website FlightAware.

That's on top of some 8,150 flights canceled over the prior five-day stretch, according to the website.

The debacle weighed on company shares and drew attention in Washington.

"The rate of cancellations and delays on Southwest Airlines is unacceptable and dramatically higher than other US carriers," the Department of Transportation said in a statement to AFP.

The agency said it expects the airline to compensate travelers for incurred losses as well as "do right" by their pilots, flight attendants and other staff, and threatened to take action against Southwest if it doesn't comply.

President Joe Biden retweeted the agency's statement, and Senator Maria Cantwell, a Washington state Democrat who chairs the Senate Commerce Committee, vowed to follow up.

"The problems at Southwest Airlines over the last several days go beyond weather," said Cantwell. "The Committee will be looking into the causes of these disruptions and its impact to consumers."

System overload

Southwest has apologized for the debacle, describing the inconvenience to customers as "unacceptable."

"We were fully staffed and prepared for the approaching holiday weekend when the severe weather swept across the continent," said a company statement Monday.

"As we continue the work to recover our operation, we have made the decision to continue operating a reduced schedule by flying roughly one third of our schedule for the next several days."

Airline officials have acknowledged that outdated systems contributed to the problems.

Southwest Chief Executive Bob Jordan alluded to a "lack of tools," adding in a December 25 message to employees that the airline is "in the process of upgrading some of those systems," according to the Wall Street Journal. 

Unions pointed to chronic underinvestment as a driver of the problems.

Lyn Montgomery, president of TWU Local 556, which represents Southwest flight attendants, posted to Twitter screenshots of flight attendants waiting for more than seven hours to receive assignments and get hotel information.

In a press release titled "Southwest Airlines Ruins Christmas for Flight Attendants," the TWU said the holiday nightmare "points to a shirking of responsibility over many years for investing in and implementing technology that could help solve for many of the issues that plague flight attendants and passengers alike."

The TWU, along with the Southwest Airlines Pilots Association (SWAPA), has been mired in labor negotiations with Southwest on a new contract.

SWAPA members picketed Southwest management outside the New York Stock Exchange earlier this month during an investor day at which executives reinstated the investor dividend "instead of focusing on the frontline employees directly responsible for the record revenues," the association said in a statement.

At the meeting, Jordan and other Southwest executives highlighted investment in a new computer system for revenue management and Jordan also expressed confidence the company would settle on contracts with unions. 

Reinstating the dividend was a priority to "restore value to our shareholders," said Jordan, who added that reviving share buybacks would have to wait. 

"We need to invest in our people," he said. "For right now, that's getting contracts done and investing in them."

Market analysis website Briefing.com said in a note that the costs for Southwest of the holiday problems "will likely be material, at least in the near-term" in higher expenses and refunds for consumers.

"However, we doubt that this debacle -- as bad as it has been -- will have a real lasting effect on Southwest," said Briefing, adding that consumers' negative feelings "will eventually fade."

Shares of Southwest fell 6.0 percent to $33.94.