U.S. air traffic controllers Tuesday missed their first full paychecks since the government shutdown began at the start of the month, while the Department of Transportation said flight delays due to staffing shortages have increased.

The controllers are facing increased financial stress and it’s getting harder to recruit much-needed workers, union officials and Transportation Secretary Sean Duffy said Tuesday. Air traffic controllers and airport security screeners are among the employees required to work during the shutdown as essential employees, even though they’re not getting regular paychecks.

“The problems are mounting daily,” said Nick Daniels, president of the National Air Traffic Controllers Association, at a news conference at New York’s LaGuardia Airport.

Duffy told reporters that 44% of the flight delays on Sunday, and about 24% of them on Monday, were due to air traffic controller staffing, compared with around 5% of the delays so far this year.

Duffy also said that the shutdown is hurting government air traffic training and recruiting, and that some funds for trainee stipends are “about to run out.”

Air traffic controller union officials have said that some members have been driving for ride-hailing platforms and taking other jobs to make ends meet.

Members of the union, including its president, plan to hand out leaflets and speak to the public at several airports across the U.S. on Tuesday, urging travelers to push Congress to end the shutdown.

The government shutdown, entering its fourth week, has added to concerns about additional strain on the U.S. air traffic control system, which has challenged airlines and travelers alike because of years of understaffing.

Flights earlier this month were delayed at several U.S. airports but the severe disruptions that preceded the end of the longest-ever shutdown, between late 2018 and early 2019, have not occurred.