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 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 travelers can avoid further disruptions and delays".
The FAA said the shortages means 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.
