Southwest Airlines cancelled 440 flights across the Midwest, Ohio Valley and the Northeast Wednesday and saw many planes delayed amid an ongoing contract dispute with its mechanics union.

An airline spokesperson said it canceled more than 350 flights in preparation for inclement weather and that it is "carefully tracking the anticipated movement of winter storms and managing out-of-service aircraft." But it acknowledged that some of its fleet is out of service due to mechanical issues.

The disruptions, recorded by flight tracking site FlightAware, come as the discount carrier spars with the Aircraft Mechanics Fraternal Association (AMFA) over a new agreement. Another 471 Southwest flights have been delayed.

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Roughly 40 of Southwest's 750 aircraft were out of service Tuesday, according to a company spokesperson, over twice the usual number of aircraft typically unavailable because of routine maintenance.

Southwest said it is investigating the service problems. On Friday, the airline declared a "state of operational emergency" to compel mechanics to return planes to service as soon as possible.

"The airline continues to operate under a staffing protocol enacted late last week to maximize availability of Mechanics across all scheduled shifts to address maintenance items so that we may safely return aircraft to service," read a statement from the airline.

In a statement to CBS News correspondent Kris Van Cleave, Southwest Chief Operating Officer Mike Van de Ven had strong words for the mechanics union saying, "AMFA has a history of work disruptions, and Southwest has two pending lawsuits against the union. We will be investigating this current disruption and exploring all possible remedies."

The mechanics union responded that "scapegoating of its expert Aircraft Maintenance Technicians does not bode well for the airline's safe operations."

The Federal Aviation Administration said that it has "heightened its oversight of Southwest," citing that as standard practice when airlines face labor issues.

Frustrations run high

Southwest faced a barrage of angry tweets Wednesday from stranded passengers.

"Southwest flight delayed and would miss connections. Booked a different flight and ran through the airport. Only to have @SouthwestAir close the gate 10 before the flight. Now I am not rebooked for 3 hours and will probably miss the hockey games #thanksfornothing," James Robinson said.

Another customer, who said she was traveling alone with a 20-month-old toddler, described being booked on a flight whose departure time was pushed back three times.

Even the Southwest employee behind its normally chirpy Twitter feed seemed irked by the surge in disgruntled customers, advising the exasperated mother to be patient and to take "a little stroll."

That’s what we did for the first hour of the delay, ha. Now pushed back to 10:55 so we’ll be going on 4hrs in the airport 🙈🤪😫 https://t.co/BuuQhebU74 — Elisabeth Givens (@elisabethholby) February 20, 2019

Southwest alluded to maintenance issues in at least one of its replies to a traveler grumbling about their cancelled trip, though it's unclear how many of the flight cancellations were linked to the labor dispute versus inclement weather.

Southwest did not immediately reply to a request for comment.

Sometimes maintenance can be more complicated than initially expected and we regret that you'll be later to your meeting. We appreciate your patience as our Team worked hard to get you going. It looks like you'll be leaving the ground any minute now. -Courtney — Southwest Airlines (@SouthwestAir) February 20, 2019

In total, airlines cancelled more than 2,200 U.S. flights Wednesday and more than 13,000 flights were delayed, according to FlightAware, as much of the country braced for a powerful winter storm.

Among other carriers, Republic had the second-most cancellations, with 240 flights suspended, while American Airlines and United cancelled 108 and 24 flights, respectively.

Reagan National airport in Arlington, Virginia, had the most disruptions --180 flights, or 39 percent of outbound trips were cancelled Wednesday.