U.S. flight reductions could escalate to as much as 20 percent if the government shutdown persists, said Transportation Secretary Sean Duffy on Friday.

Duffy answered a question about the shutdown and travel at an event hosted by Breitbart News, saying, “You might see 10 percent would have been a good number, because we might go to 15 percent or 20 percent.” Duffy later told The Hill that he was speaking theoretically.

He warned that more air traffic controllers may make “the decision to not come to work, but go take a second job” because of the funding lapse. Air traffic controllers are mandated to work through the government shutdown without pay.

The Federal Aviation Administration (FAA) said in a Thursday order that the reductions will start Friday at 4 percent and ramp up to 10 percent by Nov. 14. They are to be in effect between 6 a.m. and 10 p.m. local time and will impact all commercial airlines.

The FAA said the cuts are necessary to relieve pressure on air traffic controllers who have been working without pay for more than a month. Many are pulling six-day work weeks with mandatory overtime, and increasing numbers of them have begun calling out as the financial strain and exhaustion mount.

Meanwhile, the head of the National Transportation Safety Board (NTSB), Jennifer Homendy, wrote in an X post that the decision to cut flights at major U.S. airports is needed for safety reasons.

The board “has repeatedly stated low air traffic control staffing levels, mandatory overtime, and six-day work weeks have a direct impact on safety,” she wrote on Thursday. “Controllers are dedicated safety professionals (thank you for your continued service), but pressures are building in the system.”

Duffy and the Transportation Department, which oversees the NTSB as well as the FAA, “can’t just ignore” those safety considerations, Homendy said in the post.

“He took action to mitigate risk and ensure safety. THIS is safety management, the very foundation of our aviation system, and it’s the right thing to do,” she added.

The 40 airports selected for the slowdown by the FAA span more than two dozen states and include hubs such as Atlanta, Dallas, Denver, and Los Angeles, according to the order.

More than 800 flights were canceled nationwide, according to FlightAware, a website that tracks flight disruptions, and thousands were delayed. It’s not clear how many were due to the FAA order.

Ending the government shutdown would ease the situation for controllers, but the FAA said in its order that flight cuts will remain in place until their safety data improves.

During the shutdown, congressional Democrats are facing pressure from both unions and airline groups eager for the shutdown to end. Many Democrats have argued that strong results for Democrats in Tuesday’s election show voters want them to continue the fight until Republicans yield and agree to extend the health tax credits.

Republicans have said that they will not hold talks on health care policy until the government is reopened, while President Donald Trump has called on Senate Republicans to remove the filibuster procedural hurdle to end the shutdown.

The Epoch Times contacted the FAA for comment Friday.

The Associated Press contributed to this report.