Southwest Airlines is continuing to cancel thousands of flights day by day as it attempts to recover from what it has described as “operational challenges” over the past week.

The airline is still operating at what has been described as a “reduced schedule,” leaving thousands of passengers stranded in various parts of the country.

On Wednesday, the airline canceled 2,510 flights, or 61 percent of total flights. There were 2,914 U.S.-related canceled flights in that same day. In other words, Southwest comprised 86 percent of those cancellations.

Other U.S. airlines did not experience those same issues. Just one percent of United Airlines fights — 30 — were canceled on Wednesday. Delta also saw only 16 canceled flights, as did Frontier, according to data from FlightAware.

Meanwhile, the mass cancellations for Southwest continued into Thursday. As of 10:00 a.m. Eastern, Southwest had canceled 58 percent of its flights, or 2,362.

The mass cancellations follow a message from the airline’s CEO, Bob Jordan, who apologized, attributing the challenges largely to the winter weather and the airline’s “highly complex” system, which he admitted must be upgraded.

“Our plan for the next few days is to fly a reduced schedule and reposition our people and planes, and we’re making headway, and we’re optimistic to be back on track before next week,” he said in the message.

“We have some real work to do in making this right for now,” he added.

Transportation Secretary Pete Buttigieg reportedly spoke with Jordan, and the department said it is working with Southwest “to make sure the airline does not allow a situation like this to happen again.”