WASHINGTON (NYTIMES) - The government shutdown continues to put extraordinary pressure on the nation's air-travel system, with as many as one of every 10 transportation security officers failing to show up for work and reserve workers having to be flown in to bolster depleted ranks at some airports.

The rate of unscheduled absences of airport screening agents dropped to 7.5 per cent on Monday (Jan 21), down from 10 per cent the day before, the Transportation Security Administration said.

But the agency still had to deploy some backup officers to big airports, including Newark Liberty International in New Jersey, a spokesman for the agency said on Tuesday.

The agency's force of more than 50,000 officers learnt on Tuesday that they, like the rest of the 800,000 federal workers who have not been paid during the month-long shutdown, would miss another pay cheque this week.

The agency said that many of the absentees had cited financial troubles as their reason for not coming to work, a signal that the call-out rate is likely to continue rising until the shutdown ends.

The absentee rate for Tuesday will be available on Wednesday.

Transportation experts and elected officials have begun asking how much longer the air-travel system can continue running safely.

"Every day that goes by puts us a day or an hour closer to a potential bad thing happening," said Mr John S. Pistole, a former administrator of the TSA. "When do we hit a tipping point where there is not only a concern for the efficiency of the air-traffic system, but it becomes a safety issue?"

The entire system is operating under unusual pressures: Workers are being ordered to report for duty with no idea when they will be paid for their labour.

Airlines are losing more than US$100 million (S$135 million) a month in revenue with the grounding of federal workers. Travellers are wondering how long it will take to get through airports and whether it is still safe to fly.

On the front line stand the security screeners, who make less than US$40,000 a year and have not been paid in more than three weeks. A rising number of them have stopped showing up for work because of financial troubles, the security administration said.

In their absence, some airports have had to close checkpoints, as Baltimore-Washington International did over the weekend. At others, like Newark Liberty International and Hartsfield-Jackson Atlanta International, the TSA has had to deploy additional screeners to cope with especially long lines at checkpoints.

On Sunday, the rate of unscheduled absences for security screeners nationwide jumped to 10 per cent, more than triple what it had been a year ago, the TSA said.

Mr Pistole, who oversaw the TSA's force of more than 50,000 screening agents under former president Barack Obama, said a situation that calls for the emergency transfer of screeners between airports is "not sustainable".

He said he had passed through six airports in the last two weeks and found the officers to be in fairly good spirits. But, he said, "they're frustrated, obviously".

He said he worried that working under the stress of not being able to pay bills or feed their families could result in a dangerous lapse.

"There are a number of issues that are coming to a tipping point and, hopefully, that tipping point isn't something where a potential terrorist sees this as a chance to exploit what is perceived as a vulnerability," said Mr Pistole, who is now the president of Anderson University in Indiana.

Mr Pistole led the agency during a partial shutdown of the government that lasted 16 days in 2013. He recalled arriving at the agency's headquarters in Virginia, near the Pentagon, and finding out that he was one of just five employees, out of about 3,000 who worked there, who were considered essential.

Representative Gregory Meeks, a Democrat from New York City, said he was starting to have similar concerns about the shutdown after hearing from security officers at the city's John F. Kennedy International Airport last Saturday about "the pressure that it's starting to put on the folks at the airport".

Mr Meeks said some workers told him they were having trouble buying enough fuel to drive back and forth to Kennedy. He said he doubted that they could continue to show up much longer without getting paid.

"They may have enough to get by possibly two months, but after that it's going to get substantially worse," said Mr Meeks, who flies regularly between Kennedy and Washington.

In the meantime, he said: "You've got to consider the safety of the air-travel system in its entirety."

"I think the Port Authority is nervous," he added, referring to the Port Authority of New York and New Jersey, which operates Kennedy as well as Newark Liberty and LaGuardia airports.

Port Authority officials have been offering breaks to the federal workers on AirTrain fares and parking at the airports, an executive of the authority said. They also have offered to have airport staff pitch in on some TSA duties, such as line management and distributing the bins that travellers use during the screening process, he said.

Port Authority officials declined to speak publicly about the situation, but they said that there had been no consistent backlog at checkpoints at any of the three big airports the agency operates.

"Things are going pretty well given the circumstances," said Mr Thomas Carter, the federal security director at Newark Liberty. Staff morale, he said, was "very stable" and the airport has not had to close any of its seven checkpoints during the shutdown.