Southwest is instead following the mandate of President Joe Biden that federal contractors receive vaccinations. (Southwest contracts with the government to move employees and goods, as do most other airlines.)

Southwest told its workers they must be fully vaccinated by Dec. 8, or they risk losing their jobs.

“According to the president’s executive order, federal action supersedes any state mandate or law, and we would be expected to comply with the president’s order to remain compliant as a federal contractor,” Southwest said in a statement to Bloomberg.

It’s a further escalation of tensions for Southwest. The company’s pilots’ union has already voiced its opposition to federally mandated COVID vaccinations. The union filed suit last Friday asking a judge to temporarily block the order, saying it violated terms of the Railway Labor Act, which oversees airline-union relations.

The news comes after Southwest was forced this weekend to cancel more than 1,000 flights, with a smaller, though sizable, round of scrubbed flights Monday. The carrier blamed the problems on “disruptive weather” and air traffic control “issues.” No other carriers saw delays or cancellations anywhere close to Southwest’s numbers, however.

The carrier was flying a closer-to-normal schedule Tuesday, with only 89 cancellations, according to FlightAware. Southwest previously warned of forthcoming flight schedule cuts, saying in August that it continued to struggle with staffing issues and was attempting to better align its employee operations. Between Oct. 7 and Nov. 5, Southwest said it expects to cancel an average of 162 flights per day.