More flight cancellations frustrated travelers on the last day of 2021, and it looked almost certain it would reach hundreds of thousands over the New Year’s holiday weekend.

The airlines noted that many of the cancellations were due to staff shortages linked to a spike in COVID-19 cases, and cold weather in parts of the United States is only exacerbating the problem.

By Friday evening on the country’s east coast, airlines had canceled more than 1,550 domestic flights — 6% of scheduled flights — and about 3,500 international flights, according to tracking service FlightAware.

In doing so, the total number of cancellations in the United States exceeds 10,000 since Christmas Eve.

The cancellations come as travel numbers soar as the New Year’s weekend approaches. Since December 16, more than two million passengers have passed security checks at US airports each day, an increase of nearly 100,000 per day since November.

Led by Southwest and United Airlines, airlines have already canceled 1,500 US flights for Saturday — 700 of which correspond to Chicago’s O’Hare Airport, where forecasts announced a winter storm — and another 700 on Sunday.

Flight cancellations began to increase shortly before Christmas, especially from United Airlines, Delta Air Lines and JetBlue Airways.

On Friday, United canceled more than 200 flights, or 11% of its schedule, and that does not include cancellations from its regional subsidiary United Express. CommutAir, which operates many United Express flights, had cut a third of its schedule by noon, according to figures from FlightAware.

JetBlue canceled more than 140 flights, or 14% of its schedule, and Delta suspended more than 100, or 5%, of its flights by Friday noon.

Other forms of transportation have also been affected by the rise in coronavirus cases. The Centers for Disease Control and Prevention said Thursday that it is monitoring more than 90 cruise ships for the COVID-19 outbreak. The health agency has asked people not to travel on cruise ships, even if they have been fully vaccinated against the disease.

Residual delta variants and the emergence of a new omicron variant have raised the seven-day average of new COVID-19 cases in the United States above 350,000, about three times the rate two weeks ago, according to figures from Johns Hopkins University.