NEW YORK (AP) — Airlines continued to cancel hundreds of flights Saturday because of staffing issues tied to COVID-19, disrupting holiday celebrations during one of the busiest travel times of the year.

FlightAware, a flight-tracking website, noted nearly 1,000 canceled flights entering, leaving or inside the U.S. Saturday, up from 690 flights scrapped on Friday. Over 250 more flights were already canceled for Sunday. FlightAware does not say why flights are canceled.

Delta, United and JetBlue had all said Friday that 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,” she said of omicron’s impact on staffing. Delta and JetBlue did not respond to questions Saturday.

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

For travelers, that meant time away from loved ones, chaos at the airport and the stress of spending hours standing in line and on the phone trying to rebook flights. Peter Bockman, a retired actor, and his daughter Malaika, a college student, were supposed to be in Senegal on Saturday celebrating with relatives they hadn’t seen in a decade. But their 7:30 p.m. flight Friday from New York to Dakar was canceled, which they found out only when they got to the airport. They were there until 2 a.m. trying to rebook a flight.

“Nobody was organizing, trying to sort things out,” he said, faulting Delta for a lack of customer service. “Nobody explained anything. Not even, ‘Oh we’re so sorry, this is what we can do to help you.’”

Their new flight, for Monday evening, has a layover in Paris, and they are worried there will be issues with that one as well. They have already missed a big family get-together that was scheduled for Saturday.

FlightAware’s data shows airlines scrapped more than 6,000 flights globally for Friday, Saturday and Sunday combined as of Saturday evening, with almost one-third of affected flights to, from or within the United States. Chinese airlines made up many of the canceled flights, and Chinese airports topped FlightAware’s lists of those with most cancellations. It wasn’t clear why. China has strict pandemic control measures, including frequent lockdowns, and the government set one on Xi’an, a city of 13 million people, earlier this week.

Employees who answered the phone Sunday at customer hotlines for Air China and China Eastern airlines said they had received no notice of cancelations of flights to or from the United States.

Air China usually flies between New York City and Shanghai twice a week. China Eastern has two weekly flights to Los Angeles, one from Beijing and the other from the southern city of Shenzhen. Another Chinese airline, Hainan Airlines, suspended flights to the United States early in the pandemic.

The Civil Aviation Administration of China’s flight schedule through the end of March shows a total of 408 international flights planned per week. That is down 21% from a year earlier.

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.

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By TALI ARBEL AP Business Writer

Joe McDonald in Beijing contributed to this report.