PHILADELPHIA (Reuters) -U.S. Transportation Secretary Sean Duffy on Friday said he expects more flights to be delayed, with air traffic controllers set to miss their first paycheck as a federal government shutdown enters its 24th day.

Some 13,000 air traffic controllers and about 50,000 Transportation Security Administration officers must work without pay during the government shutdown. Controllers will miss their first full paycheck on Tuesday.

"I think as we get closer to Tuesday and then after, I think you're going to see far more disruption," Duffy said on Fox News' "America Reports."

Duffy said at a press conference at Philadelphia Airport that the Federal Aviation Administration's air traffic controller academy will run out of money within weeks to pay air traffic control students and that a few students had already decided to withdraw.

Airlines are bracing for more disruptions.

National Air Traffic Controllers Association President Nick Daniel said at the press conference that controllers are under immense stress and some are taking second jobs to pay their bills.

The shutdown "leads to an unnecessary distraction, and they cannot be 100% focused on their jobs, which makes this system less safe," Daniels said. "We didn't start a shutdown. We don't end the shutdown -- our elected officials do. And our message is simple -- end the shutdown today."

About 6.6% of flights delayed Thursday were due to air traffic controller absences -- slightly above the normal 5% -- but much lower than the 53% seen on prior days during the shutdown, the Transportation Department said.

On Thursday, the FAA said air traffic control staffing issues forced it to delay travel at airports in New York, Washington, Newark and Houston.

In 2019, during a 35-day shutdown, the number of absences by controllers and TSA officers rose as workers missed paychecks, extending wait times at some airport check points. Authorities were forced to slow air traffic in New York and Washington.

The FAA is about 3,500 air traffic controllers short of targeted staffing levels and many had been working mandatory overtime and six-day weeks even before the shutdown.