Delta Air Lines and United Airlines called on Congress Thursday to reopen the U.S. government and pay air traffic controllers, with Delta urging senators to “immediately pass a clean continuing resolution.”

U.S. air traffic controllers missed their first full paychecks on Tuesday as the government shutdown drags on through a fourth week with no end in sight while Republican and Democratic senators remain at an impasse.

“Missed paychecks only increases the stress on these essential workers, many of whom are already working mandatory overtime to keep our skies safe and secure,” Delta said in a statement Thursday.

Delta CEO Ed Bastian had warned earlier this month that the airline could see impacts from a prolonged shutdown.

Vice President JD Vance and Transportation Secretary Sean Duffy hosted a roundtable at the White House Thursday afternoon with the lobby group Airlines for America, whose members include Delta, United, American Airlines and others.

United CEO Scott Kirby told reporters outside the White House that Congress should pass a clean continuing resolution, adding that the shutdown is putting stress on the economy.

Air traffic controllers and Transportation Security Administration officers are essential employees who are required to work through the shutdown even though they are not receiving regular paychecks.

The missed paychecks come as controllers grapple with a longstanding staffing shortage. There are 3,800 fewer fully certified controllers than the FAA’s target, according to Nick Daniels, president of the National Air Traffic Controllers Association.

“These additional distractions will compound the existing risks in an already strained system,” Daniels said in an opinion piece in The Hill on Tuesday.

“Every day the shutdown continues, the National Airspace System becomes less safe than it was the day before, as the controllers’ focus shifts from their critical safety tasks to their financial uncertainty,” he said.

The shutdown began on Oct. 1 after Senate Republicans and Democrats failed to reach an agreement to keep the government open.

Democratic senators are insisting that Republicans agree to extend enhanced Affordable Care Act health insurance subsidies before they will vote for funding to reopen the government.

The Congressional Budget Office estimated Wednesday that a four-week shutdown would cost the economy at least $7 billion by the end of 2026. A six-week shutdown would cost the economy $11 billion, and an eight-week shutdown would cost $14 billion, according to CBO estimates.

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

— CNBC’s Leslie Josephs contributed to this report.