DALLAS/WASHINGTON ¨C 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 (?1.6 billion) 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.

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.