WASHINGTON (Reuters) -More than 8,000 flights were delayed across the U.S. on Sunday as air traffic controller absences continued to disrupt travel and a federal government shutdown reached its 26th day.

U.S. Transportation Secretary Sean Duffy said the Federal Aviation Administration experienced air traffic control staffing issues at 22 locations on Saturday, and added additional shortages were expected to lead to more flight delays and cancellations in the days ahead.

According to FlightAware, a flight-tracking website, there were more than 8,000 U.S. flight delays by 11 p.m. ET on Sunday (0400 GMT on Monday), an increase from about 5,300 on Saturday. Delays have often been above average since the government shutdown began on October 1. 

Southwest Airlines ( LUV ) had 45%, or 2,000 flights delayed on Sunday, while American Airlines ( AAL ) had nearly 1,200, or a third of its flights delayed, according to FlightAware. United Airlines had 24%, or 739 flights, delayed and Delta Air Lines ( DAL ) had 610 flights, or 17%, delayed.

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

Increased air travel delays and cancellations are being closely watched as observers look for indications that the shutdown is making life harder for Americans. That, in turn, could pressure lawmakers to break the budget deadlock that led to the shutdown.

The FAA on Saturday had 22 "triggers" that indicated shortages of air traffic controllers, Duffy told the Fox News "Sunday Morning Futures" program. He said the figure was "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 FAA said ground delay programs had been issued because of staffing shortages on Sunday at Chicago's O'Hare Airport, Washington's Reagan National Airport and Newark Liberty International Airport. An earlier ground stop was issued at Los Angeles International Airport, but that was later withdrawn.

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

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 checkpoints. 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 have criticized President Donald Trump and Republicans for refusing to negotiate over health care subsidies that expire at the end of the year.