WASHINGTON (Reuters) -The Federal Aviation Administration said late Sunday that air traffic control staffing issues were delaying travel at airports in Dallas, Chicago, Atlanta and Newark as a U.S. government shutdown hit its 19th day.

The FAA said numerous staffing triggers had been received for the evening shift and flights could also be delayed in Las Vegas and Phoenix because of air traffic control absences.

FlightAware said more than 5,800 flights had been delayed on Sunday. Weather issues and a Formula 1 race in Austin were also impacting flights.

More than 20% of American Airlines ( AAL ) and Southwest Airlines ( LUV ) flights were delayed Sunday, according to FlightAware.

Some 13,000 air traffic controllers and about 50,000 Transportation Security Administration officers must work during the government shutdown, but are not being paid.

Earlier this month, more than 23,000 flights were delayed over a week and Transportation Secretary Sean Duffy said 53% of flight delays were due to staffing issues, compared with 5% normally, but staffing issues have largely improved over the last week.

Air traffic control has become a flashpoint in the debate over the shutdown with both parties blaming the other. Unions and airlines have urged a quick end to the standoff.

The Trump administration is airing videos at some airport security checkpoints blaming Democrats, but many airports have refused to run them.

In 2019, during a 35-day shutdown, the number of absences by controllers and TSA officers rose as workers missed paychecks, extending checkpoint wait times at some airports. Authorities were forced to slow air traffic in New York and Washington, which put pressure on lawmakers to quickly end the standoff.

The FAA is about 3,500 air traffic controllers short of targeted staffing levels and many had been working mandatory overtime and six-day weeks even before the shutdown.