(CNN Business) – Three US airlines have been cancelled flights Right before Christmas Eve.

United Airlines (UAL) said Thursday it had had to “cancel some flights” due to the omicron type of novel coronavirus.

“The nationwide increase in omicron cases this week has had a direct impact on our crews and the people who run our operation,” said the US memo obtained by CNN.

United has canceled more than 150 flights, according to flight-tracking website FlightAware.

United said it would “notify affected customers prior to their arrival at the airport,” according to the company’s statement. “We apologize for the outage and are working hard to reschedule as many people as possible and get them on track for the holidays.”

On Thursday night, Delta Air Lines (DAL) also canceled its flights. According to FlightAware, the airline canceled more than 100 flights on Christmas Eve.

Delta said the cancellations are due to multiple problems, including the omicron variant.

“We apologize to our customers for the delay in their vacation travel plans,” Delta said in a statement. “The staff at Delta work hard to get you where you need to be, as fast and as safe as possible on the next available flight.”

In addition, Alaska Airlines said in a statement that it has canceled 17 flights due to Omicron and that more cancellations are possible on Christmas Eve.

Andy Rose and Sheriff Paget contributed to this report