Oct. 6 (UPI) -- The U.S. air traffic controllers union on Monday told its members to keep working during the government shutdown, as Transportation Secretary Sean Duffy warned that air services may be affected the longer the shutdown lasts.

Duffy told reporters on Monday that the Transportation Department has seen an increase in people calling out sick, which could make worse service issues he said will be caused by federal funds running out during the shutdown, according to the New York Times.

"If we think there's issues in the airspace, we will shut it down," Duffy said. "We will close it down, we will delay."

Duffy noted that air traffic controllers do not get paid during a federal government shutdown but still are required to work because of the essential nature of their job, and wait for back pay when the government is funded, CBS News reported.

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