US airports have been hit with widespread delays because of a shortage of air traffic controllers, who are working without pay during the federal government shutdown.

A ground stop was issued at Newark Airport on Sunday morning, a major hub servicing New York. Average delays at Newark are more than three hours and could last until Monday, the Federal Aviation Administration says.

Half of the 30 major airports in the US have staff shortages, and Transport Secretary Sean Duffy said flights will be cancelled across the national airspace "to make sure people are safe".

Nearly 13,000 air traffic controllers are working without pay as the government shutdown enters its second month.

Air traffic controllers, like other essential federal workers, are required to work without pay during the shutdown, including Transportation Security Administration (TSA) agents.

The aviation agency called on lawmakers to end the shutdown so that workers "receive the pay they've earned and travellers can avoid further disruptions and delays".

The FAA said the shortages mean it has had to reduce the flow of air traffic "to maintain safety".

"This may result in delays or cancellations," it added.

On Saturday, about 4,500 flights within the US were delayed, and more than 500 were cancelled, according to FlightAware.

In New York, the largest US city, about 80% of air traffic controllers were absent heading into the weekend, according to the FAA.

Transportation Secretary Duffy on Sunday told ABC the aviation agency would take any steps necessary to make sure travellers are safe.

"But there is a level of risk that gets injected into the system when we have a controller that's doing two jobs instead of one," he said.

He added that air traffic controllers were under a great deal of stress and working without pay.

"They don't make a lot of money, and so they may be the only person that is bringing money into the household," he said.

"They have to make a decision, do I go to work and not get a paycheque and not put food on the table? Or do I drive for Uber or DoorDash or wait tables?"

Lawmakers are at an impasse as a Republican-led bill to fund the government has failed to pass the Senate over a dozen times.

In exchange for reopening the government, Democrats are seeking to extend tax credits that make health insurance cheaper for millions of Americans.

They are also calling for a reversal of US President Donald Trump's cuts to Medicaid, a government healthcare programme used by millions of elderly, disabled and low-income people.