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

FlightAware, a flight-tracking website, noted 888 flights entering, leaving or inside the U.S. canceled Saturday, up from 690 Friday. About 200 more flights were already canceled for Sunday

Delta, United and JetBlue on Friday had all said the omicron variant was causing staffing problems leading to flight cancellations. United spokesperson Maddie King said staffing shortages were still causing cancellations and it was unclear when normal operations would return.

“This was unexpected,” King said of omicron’s impact on staffing. 

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.

Flight delays and cancellations tied to staffing shortages have been a regular problem for the U.S. airline industry this year. Airlines encouraged workers to quit in 2020, when air travel collapsed, and were caught short-staffed this year as travel recovered.

To ease staffing shortages, countries including Spain and the U.K. have reduced the length of COVID-19 quarantines by letting people return to work sooner after testing positive or being exposed to the virus.

Delta CEO Ed Bastian was among those who have called on the Biden administration to take similar steps or risk further disruptions in air travel. On Thursday, the U.S. shortened COVID-19 isolation rules for health care workers only.