US air traffic controllers are increasingly looking for second jobs and calling in sick, as they continue to work without pay during the government shutdown, the transportation secretary has said.

"The controllers are wearing thin," Sean Duffy told Fox News on Sunday, as some flights were grounded on the California coast due to staffing shortages.

After the government shut down on 1 October, air traffic controllers, who are considered essential workers, were told to keep doing their jobs without pay.

But some have been absent, leading airports to stop or slow their plane traffic, which has added to delays caused by weather and other issues.

More than 3,000 US flights were delayed on Sunday, and Duffy said he expects the situation to grow worse.

"I am seeing the stress," Duffy said in an interview on Sunday Morning Futures, adding that he had met with several controllers to hear their concerns firsthand.

"They're taking second jobs, they're out there looking, 'Can I drive Uber, can I find another source of income to make ends meet?'" he continued.

Duffy added that on Saturday, 22 alerts at airports said flights would be delayed due to a shortage of workers, one of the highest since the shutdown began.

The US government shutdown entered its 26th day on Sunday, making it the second-longest shutdown in history.

Duffy has repeatedly warned that aviation regulators would slow down the number of planes in the air if there is a shortage of safety personnel.

In his interview, he would not answer directly when asked if it was safe to fly in the US, and instead made a direct appeal to controllers.

"I need my controllers focused on the airspace, not on the finances at home," Duffy said.

On Sunday, some planes were grounded at Los Angeles International Airport due to staffing shortages, leading to disruption at the fifth-busiest airport in the country and one of the busiest in the world, as well as other flights stretching north to the city of Oakland.

The Federal Aviation Administration (FAA) alert went out at 08:45 local time on Sunday. Many flights were back on schedule before noon.

Flights at Newark Liberty International Airport, however, were delayed an average of 82 minutes due to staffing shortages, and the FAA said Dallas-Fort Worth, the second-busiest airport in the country, could experience problems from low staffing later in the day.

The FAA warned passengers in California to check with airlines about updated departure times.

The issue comes ahead of Game 3 of the World Series, which is due to be played in Los Angeles on Monday night, and about one month before the Thanksgiving holiday, one of the busiest times to fly in the US.

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 stopping 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.