A week of travel woes: More than 8,000 flights canceled since Christmas Eve

Thursday marks a week of travel woes across the country with more than 8,000 flight cancellations nationwide since Christmas Eve. As millions of Americans travel during what could be the busiest travel period since the start of the pandemic, airlines have been slammed with both winter weather and rising COVID-19 cases among flight crews.

Now, at least one major U.S. airline, JetBlue Airways, is signaling this weeklong crush of cancellations may just be the beginning.

JetBlue said on Wednesday it is preemptively canceling more than 1,200 flights over the next two weeks in order to have enough crews available to run a reliable operation.

"While the new CDC guidelines should help get crewmembers back to work sooner, and our schedule reduction and other efforts will further ease day-of cancellations, we expect the number of COVID cases in the northeast -- where most of our crewmembers are based -- to continue to surge for the next week or two," the airline said in a statement. "This means there is a high likelihood of additional cancellations until case counts start to come down."

On Thursday, more than 1,100 flights were canceled in the U.S. as of 2:30 p.m. ET.

Seattle Tacoma International has been impacted the most, holding the top spot for the most cancellations in the world for a third day in a row with nearly a third of all flights grounded. The airport added, "Staffing issues may bring further delays in operations."

Alaska Airlines, whose main hub is in Seattle, is reporting call hold times of more than 10 to 20 hours.

"We strongly urge flyers with non-essential travel scheduled before January 2, 2022, to consider changing their travel to a later date using our flexible travel policy," the airline said in a statement Wednesday.

The disruptions come as airports brace for the more than 10.5 million fliers estimated to fly from now until Jan. 3. The busiest New Year travel days are still ahead with Hopper forecasting 2.5 million could fly on Jan. 2 and Jan. 3.

Delta Air Lines is already projecting 200 to 300 daily cancellations for the upcoming weekend as "teams across [their] system continue to do all possible to mitigate constraints from increasing winter weather and the omicron variant."

Meanwhile, the Federal Aviation Administration is warning of issues of its own, such as sick air traffic controllers. The FAA warned on Thursday that weather, holiday traffic and COVID-19 "are likely to result in some travel delays in the coming days."

"Like the rest of the U.S. population, an increased number of FAA employees have tested positive for COVID-19," the FAA said in a statement. "To maintain safety, traffic volume at some facilities could be reduced, which might result in delays during busy periods."

Experts say the COVID-19-related cancellations should be a reminder to holiday fliers to protect yourself as much as you can during travel.

"These cancellations are reasons why it's important for people to wear the masks, wash their hands frequently and reduce movement around the cabin when you're on a plane," aviation expert Henry Harteveldt said. "It's just really important. All of these are part of layers of safety to keep everyone as healthy as possible while traveling for the holiday."

ABC News' Sam Sweeney contributed to this report.