Germany-based Lufthansa said Friday that it was canceling a dozen long-haul transatlantic flights over the Christmas holiday period because of a "massive rise" in sick leave among pilots. The cancellations on flights to Houston, Boston and Washington come despite a "large buffer" of additional staff for the period.

The airline says it couldn't speculate on whether COVID-19 infections or quarantines were responsible because it was not informed about the sort of illness. Passengers were booked on other flights.

Lufthansa said in a statement that "we planned a very large buffer for the vacation period. But this was not sufficient due to the high rate of people calling in sick."

U.S.-based Delta Air Lines and United Airlines said they had to cancel dozens of Christmas Eve flights because of staff shortages tied to omicron. United canceled 170 flights, and Delta called off 133, according to FlightAware.

"The nationwide spike in omicron cases this week has had a direct impact on our flight crews and the people who run our operation," United said in a statement. "As a result, we've unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport."

The airline said it was working to rebook as many people as possible.

Delta said it canceled flights Friday because of the impact of omicron and possibility of bad weather after it had "exhausted all options and resources — including rerouting and substitutions of aircraft and crews to cover scheduled flying."