WASHINGTON (Reuters) -U.S. airports reported more than 20 incidents of air traffic controller shortages on Saturday, Transportation Secretary Sean Duffy said, in the latest sign of the government shutdown's impact.

Some 13,000 air traffic controllers and about 50,000 Transportation Security Administration officers must work even if they are not being paid during the shutdown, which started on October 1 as lawmakers deadlocked over the federal budget.

The Trump administration has warned that flight disruptions will increase as controllers miss their first full paycheck on Tuesday.

The Federal Aviation Administration on Saturday had 22 "triggers" that indicated shortages of air traffic controllers, Duffy told Fox News' "Sunday Morning Futures" program. He called that figure "one of the highest that we've seen in the system" since October 1. 

"That's a sign that the controllers are wearing thin," Duffy said. 

The state of air safety has been closely watched for signs of an increase in delays or cancellations, as observers look for anything that shows the shutdown is making life harder for Americans, which in turn could pressure lawmakers to break the deadlock.

According to FlightAware, a flight tracking website, there were more than 5,300 U.S. flight delays on Saturday and more than 2,500 by 12 p.m. ET on Sunday. Delays have often been above average since the shutdown began. 

With no end in sight for the federal shutdown, Duffy predicted more traffic controller shortages that would lead to more flight delays and cancellations over the next few days.

The FAA said air traffic control staffing issues had already been reported on Sunday for Chicago and Newark flights. The agency issued a ground stop at Los Angeles International Airport due to traffic controller staffing shortages at around 11:30 a.m. ET (1530 GMT). 

By noon ET on Sunday, the agency had recorded shortfalls in air traffic controllers at six U.S. airports.

Air traffic controllers received a paycheck two weeks ago at 90% of their regular pay. But Tuesday's payday would have been for their first pay period solely for work in October.  

Controllers facing the prospect of missing a federal paycheck are looking for other sources of income, Duffy said. 

"They're taking second jobs, they're out there looking," he said. 

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.

In 2019, during a 35-day shutdown, the number of absences by controllers and TSA officers rose as workers missed paychecks, extending wait times at some airport check points. Authorities were forced to slow air traffic in New York and Washington.

Duffy and other Republicans have criticized Democrats for opposing a "clean" short-term funding bill with no strings attached. Democrats criticize President Donald Trump and Republicans for refusing to negotiate over health care subsidies that expire at the end of the year.