Hampered by staffing shortages tied to COVID-19, airlines canceled or delayed over 800 flights in and out of Denver International Airport by Sunday evening as a nationwide surge in omicron cases continued to disrupt travel during the busy holiday weekend.

As of 8:45 p.m. Sunday, 684 flights were delayed and 123 flights were canceled at the airport, according to Flight Aware. On Saturday, 283 flights were delayed and 67 flights were canceled.

By Sunday evening, Denver had the most flight delays of any airport in the world for both destination and origin airports, according to Flight Aware. Denver also had the fourth most flight cancellations in the world.

Airport officials said the delays and cancellations weren’t a result of the airport itself, saying the blame likely fell on the airlines.

“We don’t have any control over that right now,” Stephanie Figueroa with Denver International Airport said Sunday.

Of the 684 delays at Denver International Airport, 213 were Southwest flights and 199 were United Airlines flights.

In a statement, United Airlines said the spike in omicron cases impacted its flight crews over the Christmas weekend.

“We’ve unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport,” the statement said. “We’re sorry for the disruption and are working hard to rebook as many people as possible and get them on their way for the holidays.”

Other airlines including Delta and Alaska Airlines have also attributed weekend delays and cancellations to the omicron variant.

Southwest said its delays were “mostly related to weather challenges being experienced across parts of the country today.”

Sunday marked the third day of mass flight cancellations throughout the U.S. On Saturday, 3,584 U.S. flights were delayed and 998 were canceled, according to Flight Aware. As of 8:45 p.m. Sunday, 6,134 flights were delayed and 1,435 were canceled.

The omicron variant is now the dominant strain of COVID-19 in the U.S., accounting for about 73% of cases as of early last week, according to the Centers for Disease Control and Prevention.

On Saturday, the U.S.’s seven-day average of COVID-19 cases hit 201,330 — the country’s highest seven-day average since January, according to data collected by The New York Times.