U.S. airlines canceled more than 2,100 flights on Sunday as Transportation Secretary Sean Duffy warned that air traffic across the nation could "slow to a trickle" if the federal government shutdown lingers into the busy Thanksgiving travel holiday season.

The slowdown at 40 of the nation`s busiest airports is now in its third day and beginning to cause more widespread disruptions. The FAA last week ordered flight cuts at the nation`s busiest airports as some air traffic controllers, who have gone unpaid for nearly a month, have stopped showing up for work.

In addition, some 7,000 flight delays were reported on Sunday alone, according to FlightAware, a website that tracks air travel disruptions. More than 1,000 flights were canceled Friday, and more than 1,500 on Saturday.

The FAA reductions started Friday at 4% and will increase to 10% by Nov. 14. They are in effect from 6 a.m. to 10 p.m. local time and will impact all commercial airlines.

Hartsfield-Jackson International Airport in Atlanta had the most cancellations Sunday, with more than 570, followed by Newark Liberty International Airport in New Jersey, with at least 265. In Georgia, weather could also be a factor, with the National Weather Service office in Atlanta warning of widespread freezing conditions through Tuesday.

Traveler Kyra March finally arrived at Hartsfield-Jackson on Sunday after a series of postponements the day before.

"I was coming from Tampa and that flight got delayed, delayed, delayed. Then it was canceled and then rebooked. And so I had to stay at a hotel and then came back this morning," she said.

The FAA said staffing shortages at Newark and LaGuardia Airport in New York were leading to average departure delays of about 75 minutes.

Detroit Metropolitan Wayne County Airport in Michigan was mostly empty Sunday morning, with minimal wait times at security checkpoints as delays and cancellations filled the departures and arrivals boards.

Earlier Sunday, Duffy warned that U.S. air traffic could decline significantly if the shutdown persists. He said additional flight cuts - perhaps up to 20% - might be needed, particularly after controllers receive no pay for a second straight pay period.

"More controllers aren`t coming to work day by day, the further they go without a paycheck," Duffy told "Fox News Sunday."

And he prepared Americans for what they could face during the busy Thanksgiving holiday.

"As I look two weeks out, as we get closer to Thanksgiving travel, I think what`s going to happen is you`re going to have air travel slow to a trickle as everyone wants to travel to see their families," Duffy said.

With "very few" controllers working, "you`ll have a few flights taking off and landing" and thousands of cancellations, he said.

"You`re going to have massive disruption. I think a lot of angry Americans. I think we have to be honest about where this is going. It doesn`t get better," Duffy said. "It gets worse until these air traffic controllers are going to be paid."

The government has been short of air traffic controllers for years, and multiple presidential administrations have tried to convince retirement-age controllers to remain on the job. Duffy said the shutdown has exacerbated the problem, leading some air traffic controllers to speed up their retirements.

"Up to 15 or 20 a day are retiring," Duffy said on CNN.

Duffy said Defense Secretary Pete Hegseth texted him with an offer to lend military air traffic controllers, but it`s unclear whether the staff is certified to work on civilian systems.

Duffy denied Democratic charges that the flight cancellations are a political tactic, saying they were necessary due to increasing near-misses from an overtaxed system.

"I needed to take action to keep people safe," Duffy said. "I`m doing what I can in a mess that Democrats have put in my lap."

Airlines for America, a trade group representing U.S. carriers, said air traffic control staffing-related delays exceeded 3,000 hours on Saturday, the highest of the shutdown, and that staffing problems contributed to 71% of delay time.