United Airlines CEO Scott Kirby said the federal government shutdown could hurt bookings if it continues.

Despite the funding impasse, essential federal employees including Transportation Security Administration officers and air traffic controllers are required to work without pay. The shutdown started Oct. 1 as Congress failed to pass a funding bill.

Speaking on an earnings call on Thursday, Kirby said the shutdown so far hasn’t impacted the carrier’s business.

“I think that at least for the first couple of weeks, people thought it was going to get resolved, so they just kind of continued business as usual,” he said. “But as time goes on, as people read headlines and say, ‘it’s not going to get resolved soon.’ People start to lose confidence in the government and the government’s ability to resolve this. And that’s going to start to impact bookings.”

Kirby said there isn’t an exact cutoff for when the airline could start to see an impact, but added “every day that goes by, the risk to the U.S. economy grows. So I hope we will avoid an unforced error here.”

Delta Air Lines CEO Ed Bastian last week made a similar warning about how a prolonged shutdown could affect air travel but emphasized that the airline’s operation hadn’t been affected.

The shutdown has raised concerns about already thin-staffing among air traffic controllers, the Federal Aviation Administration said. Insufficient staffing at some FAA facilities last week disrupted flights at airports, including in Nashville, Tennessee, and Burbank, California.

Members of the union that represents U.S. air traffic controllers handed out leaflets outside of LaGuardia Airport in New York, as well as in Washington, D.C., and Chicago on Tuesday to urge the public to ask lawmakers to end the shutdown.

A more-than-monthlong shutdown starting in late 2018 ended hours after a shortage of air traffic controllers snarled air travel in the New York area.