WASHINGTON (Reuters) -The U.S. Federal Aviation Administration said on Friday it was delaying flights at eight airports including in Atlanta, San Francisco, Houston, Washington and Newark, citing widespread air traffic staffing issues as controllers have not been paid during the government shutdown.

The FAA said it has staffing shortages at 10 locations across the country, which could potentially result in delays at more airports. 

The agency has separately required airlines to cancel 4% of flights at 40 high-volume airports to address air traffic control staffing -- or more than 700 flights in total.

Under FAA ground delay programs, flights are being delayed by an average of 83 minutes at Reagan Washington National, 66 minutes at San Francisco, 52 minutes at Newark, 47 minutes at Austin, and 80 minutes at Houston Bush.

More than 2,300 flights had been delayed as of 12:45 p.m. EST, according to Flightwire, a flight tracking site. Some airlines have expressed concern that controllers, who have not been paid during the shutdown, might feel emboldened not to show up for work because of the flight reductions.

The 38-day shutdown, the longest in U.S. history, has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration agents to work without pay.

In an interview Friday, Transportation Secretary Sean Duffy said he had asked the controllers union to reach out to controllers to ask them to come to work.

"I need them to come to work," Duffy said. "This is not about you now having a 'get-out-of-jail-free' to not come to work."

The FAA said this week that 20% to 40% of controllers daily were not showing up for work.

On Oct. 31, the FAA said nearly half of the 30 busiest U.S. airports faced shortages of air traffic controllers, leading to more than 6,200 flights being delayed and 500 canceled, in the single worst day since the shutdown began. In New York 80% of air traffic controllers were absent.