US air travellers are experiencing a second day of flight delays and cancellations, as the government shutdown causes a worsening shortage of air traffic controllers.

On Tuesday evening, the flight delays hit even more airports, with the cities of Nashville, Dallas, Chicago, and Newark especially affected.

On Monday, air traffic control at the Hollywood Burbank Airport in California was unmanned for nearly six hours and remotely managed due to staffing shortages.

Transportation Secretary Sean Duffy said some air traffic controllers are calling in sick as the shutdown, now in its seventh day, adds stress on workers. He estimated that staffing has at times dropped to 50% in some areas.

Air traffic controllers are considered essential workers and are expected to work without pay during government shutdowns.

About 3,200 US flights were delayed as of Tuesday afternoon, according to FlightAware.

On Monday, staffing issues affected flights at Newark, Phoenix, Denver, Las Vegas and Burbank Airport, causing anywhere from 40-minute to 60-minute delays, according to flight data.

On Tuesday evening, the Federal Aviation Administration (FAA) issued a Ground Delay at Chicago's O'Hare airport. It is unclear when the delay is scheduled to end.

Nashville and Dallas were also experiencing delays on Tuesday, according to the FAA.

Nashville International Airport posted a notice on Tuesday indicating that all flights coming in and out of the city would be "reduced" due to a shortage of air traffic controllers. The airport said this would cause delays but the reductions would continue "until further notice".

Drew MacQueen, a union official with the National Air Traffic Controllers Association, told NewsNation that his colleagues are over-worked, and over-stressed from not being paid.

"Air traffic controllers didn't start the shutdown and certainly air traffic controllers aren't going to end the shutdown. That's up to the politicians," MacQueen said.

"What they don't need is the fatigue of dealing with that job while being short staffed, working six days a week, 10 hours a day," he continued. "Now they're worried about when they'll get paid."

Tuesday marked the second day of airport delays caused by shutdown-related staffing problems.

The shutdown began on 1 October after Republican and Democratic lawmakers failed to agree on resolving a budget dispute. It's unclear when they will be able to break the deadlock in Congress.

Some 40% of the federal workforce - about 750,000 people - have been placed on unpaid leave during the shutdown, while essential government workers are expected to keep working without pay.

The National Air Traffic Controllers Association, which represents more than 20,000 air traffic controllers, engineers and professionals in aviation safety, warned employees that failure to show up could lead to termination.

"Participating in a job action could result in removal from federal service," the union posted on its website. "It is not only illegal, but it also undermines NATCA's credibility and severely weakens our ability to effectively advocate for you and your families."

The warning came after Transportation Secretary Duffy said at a news conference on Monday that there has been a "slight tick" in air traffic controllers calling in sick.

"You'll see delays come from that," he said, adding that safety is the priority even if it means some delays or cancellations.

He said air traffic controllers should be able to focus on work and not have to worry about a paycheque.

"Now what they think about as they're controlling our airspace is: 'How am I going to pay my mortgage?'" he said.

Speaking to Fox News, Duffy said that regulators would slow down the number of planes in the air, due to the shortage of safety personnel.

"If we don't have controllers, we're going to make sure the airspace is safe. So what we do is we'll slow traffic," Duffy said on Tuesday.

Strain on US air travel helped end a government shutdown in 2019. At the time, the shutdown had entered its fifth week and disrupted airline operations, including pilot training.

It prompted several air traffic controllers to stay home on the same day on 25 January 2019, temporarily shutting down travel at New York's LaGuardia airport.

That same day, Donald Trump, who at the time was in his first term as president, agreed to sign a short-term spending bill, effectively ending the government shutdown after 35 days.