The Federal Aviation Administration said on Tuesday that air traffic control staffing issues were delaying flights at Houston and Newark airports on the 21st day of a U.S. government shutdown.

The FAA issued ground stops impacting flights at Houston’s George Bush Intercontinental Airport and Hobby Airport because of absences from controllers.

Houston Bush had also faced delays earlier when communications issues triggered a ground stop that was later extended because of staffing issues.

More than 163 flights at Houston Bush had been delayed by 7 p.m. ET (2300 GMT), or about 12% of flights, according to flight tracking service FlightAware. Another 53 flights at Hobby were delayed, or 8% of flights. More than 171 flights at Newark Liberty International in New Jersey, or about 15%, were also delayed.

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.

National Transportation Safety Board chair Jennifer Homendy said on Tuesday she was concerned about the shutdown impact on controllers. “This is a really hard job, and you have to be really tuned in,” Homendy said, adding controllers can get distracted when not getting paid and worrying about personal financial issues.

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.

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.

In 2019, during a 35-day government 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.