NEWARK, N.J. — The six-day-old federal government shutdown is straining the country’s air traffic control system, Transportation Secretary Sean Duffy said Monday. While the national airspace remains safe, Duffy said, he expressed concern that air traffic controllers are becoming distracted by the stress of not receiving a paycheck and may even take on other jobs.

Air traffic controllers are required to work without pay during government shutdowns because the Department of Transportation considers them essential workers to maintain flight safety.

“This shutdown has put way more stress on our controllers,” Duffy said during a press conference Monday at Newark Liberty International Airport, where he met with air traffic controllers who told him they were worried about paying their mortgage,

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