New York: US carrier Southwest Airlines announced on Monday that it had canceled hundreds more flights following a major weekend service disruption.
The airline said weather and air traffic control issues were behind the cancellation of over 1,085 flights on Sunday, accounting for 29% of its schedule. Some 900 other flights were delayed.
It was the highest rate of cancellations by any airline, with competitors reporting no issues.
Southwest said that weather challenges at airports in the US state of Florida early last weekend, compounded by unexpected air traffic control issues, triggered delays and prompted significant cancellations.
"We've continued diligent work throughout the weekend to reset our operation with a focus on getting aircraft and crews repositioned to take care of our customers," the airline said.
But the US Federal Aviation Administration (FAA) denied on Sunday that air traffic control disruptions were taking place, while confirming severe weather did trigger some momentary flight delays and cancellations on Friday.
"Flight delays & cancellations occurred for a few hours Friday PM due to widespread severe weather, military training, & limited staffing in one area of the Jacksonville en route center," the agency wrote in a tweet.
The Dallas-based carrier's flight disruptions come in the midst of a dispute with its pilots' union, which recently asked a federal court to block the airline's order that all employees get vaccinated against COVID-19 by December 8.
This sparked speculation about whether a labor stoppage in opposition to the vaccine requirement was behind the cancellations.
But the Southwest Airlines Pilots Association has denied that its members were conducting a sickout or slowdown to protest the vaccine mandate, adding that the organization "has not authorized, and will not condone, any job action.''
The union, however, did say that an "already strained relationship" existed between it and the company, and that operations at Southwest had "become brittle and subject to massive failures under the slightest pressure.''
"Our operation and our frontline employees have endured continuous and unending disruptions since the first time our airline made headlines in early June due to widespread IT failures. Our pilots are tired and frustrated because our operation is running on empty due to a lack of support from the company," a union statement read.
The airline has struggled all summer with high numbers of delayed and canceled flights. It had announced in August that it would reduce its September and October schedule.