U.S. airline bosses are stepping up their criticism of the partial government shutdown, warning that the closing threatens to snarl air travel as it drags into its 34th day.

“We are close to a tipping point as employees are about to miss a second paycheck,’’ JetBlue Airways Corp. Chief Executive Officer Robin Hayes said on a conference call Thursday with analysts and investors. “The longer this goes on, the longer it will take for the nation’s air travel system to rebound.’’

CEOs at larger carriers backed him up, with American Airlines Group Inc.’s Doug Parker warning of “long lines’’ and “delayed airspace.’’ Southwest Airlines Co. said it lost out on as much as $15 million in sales this month because of the shutdown.

“We’d be crazy not to be concerned about this,” Southwest CEO Gary Kelly said in an interview. “Common sense tells us the longer this goes on, the more problems are likely.”

The Federal Aviation Administration said it hadn’t seen any impact on air-traffic control in the last few weeks.

“We have not observed any appreciable difference in performance over the last several weeks compared to the same periods during the previous two years,’’ the FAA said in a statement.

Union Concern

The rising alarm from airline CEOs echoed complaints this week from unions representing pilots, flight attendants and air-traffic controllers.

“We have a growing concern for the safety and security of our members, our airlines, and the traveling public due to the government shutdown,’’ a union coalition said in a statement late Wednesday.

“Air traffic controllers, transportation security officers, safety inspectors, air marshals, federal law enforcement officers, FBI agents, and many other critical workers have been working without pay for over a month,” the coalition said. The statement was released by the Air Line Pilots Association, the Association of Flight Attendants-CWA and the National Air Traffic Controllers Association.