Airlines continued to cancel hundreds of flights as staffing issues tied to COVID-19 disrupted holiday celebrations during one of the busiest travel times of the year, writes Los Angeles Times.

FlightAware, a flight-tracking website, noted nearly 1,000 flights entering, leaving or inside the United States canceled Saturday, up from 690 Friday. More than 250 more flights were already canceled for Sunday. FlightAware does not say why flights are canceled.

According to FlightAware, the three airlines canceled more than 10% of their Saturday scheduled flights. American Airlines also canceled 90 flights Saturday, about 3% of its schedule, according to FlightAware. American spokesperson Derek Walls said the cancellations stemmed from “COVID-related sick calls” and the airline contacted customers on Friday. European and Australian airlines have also canceled holiday-season flights due to staffing problems tied to COVID-19.