A shortage of air traffic controllers could delay more flights, the Federal Aviation Administration warned on Wednesday, as concerns grow about the effect of the government shutdown on U.S. aviation.

About 10,000 flights were delayed on Monday and Tuesday, though disruptions dropped on Wednesday to just over 2,500. Delays could be caused by weather or other reasons aside from staffing issues. But a shortfall of already-thin air traffic control staffing this week had prompted the FAA to slow or halt arrivals in Burbank, California, and Nashville, Tennessee, among others.

Transportation Secretary Sean Duffy warned Monday that the FAA is seeing a “slight uptick” in sick calls of air traffic controllers.

He also said the shutdown is exacerbating concerns about the strain on air traffic controllers, a shortage of whom has vexed airline executives for years.

“Nearly 11,000 fully certified controllers remain on the job, many working 10-hour shifts as many as six days a week, showing extraordinary dedication to safely guiding millions of passengers to their destinations—all without getting paid during this shutdown,” the air traffic controllers’ union, the National Air Traffic Controllers Association, said in a statement.

Earlier Wednesday, the FAA had warned there could be a staffing trigger at Newark Liberty International Airport, but that caution had been removed by the afternoon. Newark was not seeing an influx of flight delays.

The government shutdown stretched into its eighth day Wednesday, as the Senate failed to pass a funding proposal again.

Duffy said Tuesday on CNBC’s “Squawk Box” that no one airport is consistently having issues with air traffic controller staffing and urged Congress to pass a funding bill.

“The conversations and the fights that are happening are about issues allegedly that are going to come to fruition at the end of the year,” he said. “Open up the government, take the months to have a conversation and negotiate, but let’s not use this as leverage with hardworking Americans that keep our skies safe for political benefit.”

During a shutdown, “essential” workers such as air traffic controllers and TSA agents are continuing to work without pay, while many other employees are placed on furlough.

A more than monthlong shutdown that started in late 2018 ended early the next year, hours after a shortage of air traffic controllers snarled air travel in New York.