Air travel could face more disruptions as the government shutdown enters its 20th day, with air traffic controllers starting to exhibit "frustration" at their delayed compensation , Transportation Secretary Sean Duffy told Fox News' Fox & Friends and CNBC's Squawk Box Monday.

Some 13,000 air traffic controllers, charged with patrolling the skies and directing plane traffic, are among those workers looking for alternative ways to earn cash while they go without paychecks during the lapse in funding, according to Duffy.

They are due to be paid again Oct. 28, but they are at risk of missing that paycheck if the government shutdown persists, Duffy said. Because the Federal Aviation Administration defines air traffic controllers as essential workers , they are required to st

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