WASHINGTON (AP) — On Sunday, a staggering 2,700 flights were canceled by U.S. airlines, as Transportation Secretary Sean Duffy cautioned travelers that a prolonged federal government shutdown could severely disrupt air traffic, especially as the Thanksgiving travel rush approaches.

The third day of the slowdown has already begun to ripple across the nation’s aviation network, affecting 40 major airports. This follows an FAA mandate last week to reduce flights at these key locations, prompted by a growing number of air traffic controllers failing to report for duty after nearly a month without pay.

According to FlightAware, a platform that monitors flight disruptions, Sunday alone saw nearly 10,000 flight delays. The situation has worsened since Friday when over 1,000 flights were canceled, followed by more than 1,500 on Saturday.

The FAA’s flight reductions began at 4% last Friday and are expected to reach 10% by November 14. These reductions are scheduled to take place daily from 6 a.m. to 10 p.m. local time, affecting all commercial airlines.

On Sunday, Hartsfield-Jackson International Airport in Atlanta faced the highest number of cancellations, with Chicago O’Hare International close behind, as both airports grappled with challenging weather conditions. The National Weather Service in Atlanta has issued a warning for widespread freezing conditions that could persist through Tuesday, adding another layer of complexity to the travel turmoil.

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 persisted. He said additional flight cuts perhaps up to 20% might be needed, particularly if 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.