Nearly 50% of the 30 busiest US airports faced shortages of air traffic controllers, the Federal Aviation Administration said on Friday, leading to flight delays nationwide as a federal government shutdown hit its 31st day.

The absence of controllers on Friday is by far the most widespread since the shutdown began, with one of the worst-hit regions being New York, where 80% of air traffic controllers were out, the agency said.

At least 35 FAA facilities, including several at the largest US airports, reported staffing problems. Airports affected included facilities in New York, Austin, Newark, Phoenix, Washington, Nashville, Dallas and Denver. At some airports, delays averaged one hour or more.

The shutdown has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration officers to work without pay.

"After 31 days without pay, air traffic controllers are under immense stress and fatigue," the FAA said late Friday.

"The shutdown must end so that these controllers receive the pay they've earned and travelers can avoid further disruptions and delays," it added.

The impact on the system would have been far worse on a typical Friday. However, Halloween evening traffic was 20% lower than usual, which helped mitigate the effects of staffing shortages, airline officials said.

More than 5,600 flights were delayed on Friday and 500 canceled, according to FlightAware, a flight tracking website.

At New York's LaGuardia Airport, 50% of flights were delayed and 12% canceled, with delays averaging 140 minutes, while Washington, DC's Reagan National Airport had a quarter of flights delayed.

"Coming into this weekend and then the week after, I think you are going to see even more disruptions in the airspace," US Transportation Secretary Sean Duffy said on Fox News' "America's Newsroom."