Unions representing air-traffic controllers, airline pilots and flight attendants issued a joint statement Wednesday warning of "serious safety concerns" in the air travel industry if the partial government shutdown continues.

The shutdown has forced air-traffic controllers, air marshals, Transportation Security Administration officers, FBI agents and other safety staff to work without pay, the statement says.

"In our risk-averse industry, we cannot even calculate the level of risk currently at play, nor predict the point at which the entire system will break. It is unprecedented," reads the statement issued by the presidents of the National Air Traffic Controllers Association, Air Line Pilots Association and the Association of Flight Attendants.

The Federal Aviation Administration responded to the unions' concerns in a Wednesday statement to USA TODAY, stressing that U.S. air travel remains safe and effective:

"The FAA continually reviews and analyzes the performance of the national airspace system to assess its safety and efficiency," a statement issued by an FAA spokesperson reads.

"We have not observed any appreciable difference in performance over the last several weeks compared to the same periods during the previous two years. We remain grateful to the air-traffic controllers for their professional and dedication to their safety mission."

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The FAA says it has seen no operational disruptions and "no unusual increased absenteeism." The statement says the agency has updated its shutdown plan to recall inspectors and engineers due to safety concerns.

In a Wednesday night appearance on CNN's "Cuomo Prime Time," National Air Traffic Controllers Association President Paul Rinaldi lamented staff shortages across the country that are requiring some controllers to work up to 60 hours a week without pay.

"The government needs to open now," Rinaldi said.

He expressed concern that fatigued employees worrying about personal finances could make dangerous mistakes: “It can’t go on for months.”

The unions' joint statement specifically cites air-traffic controller staffing as an issue, saying that a large number of controllers who are eligible to retire have little incentive to keep working without a paycheck.

"When they elect to retire, the National Airspace System (NAS) will be crippled," the statement says.

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Air-travel worries have been a high-profile concern as the historic partial government shutdown approaches its fifth week.

Congress and the White House are locked in an impasse that has led to the longest government shutdown on record over President Donald Trump's demands for $5.7 billion to build a wall along the U.S.-Mexico border. Trump has threatened to veto any legislation that doesn't provide money for the structure. Democrats, who oppose the wall, are refusing to give it to him.

Affected federal employees number 800,000; they will miss their second paycheck this week. About half of those employees, including about 50,000 airport security workers, are considered "essential" and are working anyway.

An increasing number of TSA workers have been calling in sick, with callouts reaching 10 percent this week.

The agency, which provides security at the nation's airports, acknowledged "many employees are reporting that they are not able to report to work due to financial limitations." However, national average wait times remain within normal TSA times, the agency said in a statement.

Earlier this month, dozens of aviation industry groups urged Trump and Congress to end the shutdown or risk travel troubles.

"This partial shutdown has already inflicted real damage to our nation’s aviation system and the impacts will only worsen over time,'' a letter from a coalition of aviation industry groups said. "We urge you to act quickly to resolve these issues.''

Contributing: Steve Kiggins, Eliza Collins, Michael Collins, John Bacon and Dawn Gilbertson