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The US government is launching an investigation into Southwest Airlines after mass flight cancellations left travellers stranded last month.

The Department of Transportation is in the initial phase of a "comprehensive investigation into Southwest Airlines' holiday debacle that stranded millions," a spokesperson said.

Southwest said it was co-operating with the investigation.

"I can't say it enough. We messed up," CEO Bob Jordan told CNBC.

"We took good will out of the bank. We know that," he said. "We have work to do to repair trust, but our customers are very loyal and we're seeing that loyalty."

The airline cancelled more than 16,000 flights during a busy week of holiday travel last month, leaving many people stranded.

The transport department is investigating whether the airline knowingly scheduled more flights in late December than it could realistically handle. It said doing so would be considered a deceptive practice under federal law.

It said it had also "made clear to Southwest that it must provide timely refunds and reimbursements" to compensate passengers.

The flight disruptions came after a huge winter storm hit the US and Canada, forcing several airlines to cancel flights.

In an earlier statement, the airline said its holiday flight schedule had been "thoughtfully designed and offered to our customers with the backing of a solid plan to operate it, and with ample staffing".

The company argued an "unprecedented storm" put pressure on its systems.

On Thursday, Southwest said the travel chaos had cost it $800m (£645.5m).

Bookings have also slowed this month after the travel chaos, the company said in its quarterly earnings report, though it added it was seeing strong bookings for March.