MIAMI – Southwest Airlines passengers continue to deal with flight delays across the country.

Early Monday morning, 336 flights had already been canceled by Southwest, with another 175 of the carrier’s flights delayed.

The airline canceled more than 1,000 flights on Sunday after having around 800 nationwide cancellations on Saturday.

Miami International Airport and Fort Lauderdale-Hollywood International Airport are among the sites affected.

Southwest said that air traffic control issues and disruptive weather are to blame for many of the flights being canceled or delayed.

The company released a statement that read, in part:

“We experienced weather challenges in our Florida airports at the beginning of the weekend, challenges that were compounded by unexpected air traffic control issues in the same region, triggering delays and prompting significant cancellations for us… We’re working diligently to accommodate our customers as quickly as possible… We apologize, and we again thank everyone for patience as we work first to be safe, and second to be as quick as possible in solving disrupted plans.”

The Federal Aviation Administration, however, says that no major air traffic control issues have been reported since Friday. In a tweet, the FAA said some airlines continue to experience scheduling challenges due to aircraft and crews being out of place.

The airline’s pilot union, the Southwest Airlines Pilots Association, said: “Our pilots will continue to overcome SWA management’s poor planning, as well as any external operational challenges.”

Henry H. Harteveldt, president and travel industry analyst at The Atmosphere Research Group, based in San Francisco, points to other causes for the cancellations.

First, he says Southwest has scheduled more flights than it can handle, a problem that started in June. He also noted that Southwest operates what’s known as a point-to-point route network, and when a delay occurs, it “cascades” along the remaining flight segments. That’s because, for example, a Southwest flight departing Fort Lauderdale for the airline’s home base of Dallas may make multiple stops along the way.

But Harteveldt says the most troubling reason is the likelihood that some pilots who oppose Southwest’s decision to mandate COVID-19 vaccinations are participating in an illegal job action where they call in sick or are engaging in a “work slowdown.”

In a statement Saturday, the Southwest Airlines Pilots Association, said that’s not the case.

“SWAPA is aware of operational difficulties affecting Southwest Airlines today due to a number of issues, but we can say with confidence that our pilots are not participating in any official or unofficial job actions,” it said.

Harteveldt noted Southwest’s woes could linger and affect its fourth-quarter financial performance.

“All of this is happening as people are in the midst of booking their Thanksgiving and Christmas/New Year holiday travel,” he said. “It’s very possible that some people who might normally book on Southwest may see this news and choose to fly other airlines.”

The airline tweeted out that affected travelers should explore self-service rebooking options on their website.