More than 4,000 flights were delayed as holiday travel spikes in the U.S.

Thousands of flights were delayed — and a number of others canceled — across airports in the United States on one of the busiest travel days of the year. More than 4,200 flights were delayed and another 109 cancelled as of Sunday afternoon, according to FlightAware, with numbers continuing to climb.

The flight cancellations and delays come during a period of chaotic travel said to resemble pre-pandemic levels. Airlines and airport staff were preparing as an estimated 4.5 million Americans were expected to fly this Thanksgiving holiday, according to AAA.

In preparation for the holiday rush, earlier this month the Transportation Security Administration announced it would cut the cost of enrolling into TSA PreCheck, a program that gives passengers access to expedited lines.

Airports are also warning travelers about delays and other disruptions to their travel.

"We can't stress this enough - plan ahead and arrive early," New York City's John F. Kennedy International Airport tweeted on Sunday. "Budget extra travel time to arrive at JFK Airport, park in your reserved spot, check in, and get through security."

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