WASHINGTON (AP) — On Sunday, over 2,100 U.S. flights were grounded, primarily due to the ongoing government shutdown and a directive from the Federal Aviation Administration (FAA) to curtail air traffic.

Now entering its third day, the reduction in air traffic at 40 of America’s busiest airports is causing increasingly widespread disruptions. On Sunday alone, more than 7,000 flights were delayed, as reported by FlightAware, a site that monitors air travel disruptions.

In the days leading up to Sunday, the situation had already seen the cancellation of over 1,000 flights on Friday and an additional 1,500 on Saturday.

Earlier in the day, U.S. Transportation Secretary Sean Duffy issued a warning that the nation’s air traffic might “slow to a trickle” if the federal government shutdown persists into the peak Thanksgiving travel period.

The FAA’s decision last week to reduce flights at major airports came as a response to numerous air traffic controllers, unpaid for nearly a month, opting not to report for work.

The 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.

On Sunday, more than 2,100 flights were canceled and more than 7,000 flights were delayed, according to the aviation tracking website FlightAware. That follows more than 1,500 cancellations on Saturday.

Hartsfield-Jackson International Airport in Atlanta had the most cancellations Sunday, with 173, followed by Newark Liberty International Airport in New Jersey, with 115.

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.

Duffy has 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.

From Oct. 1 to Nov. 7, controller shortages have disrupted more than 4 million passengers on U.S. carriers, according to Airlines for America.