US Transportation Secretary Sean Duffy has warned that air travel capacity will be cut by up to 10% at 40 major airports over the coming days, if the government shutdown continues.

The decision, which is expected to affect domestic flights only, was made because air traffic controllers had been reporting fatigue, the head of the Federal Aviation Administration (FAA) said at a briefing alongside Duffy.

Some 1.4 million federal workers, from air traffic controllers to park wardens, are working without pay - or are on forced leave - because the US Congress has not agreed on a federal funding budget.

Unions say many employees are becoming ill with stress or are being forced into taking second jobs.

On Wednesday, the impasse became the longest shutdown in US history.

It is unusual, said FAA chief Bryan Bedford of the planned flight reductions, just as the shutdown is unusual, just as the fact that our controllers haven't been paid for a month is unusual.

The flight reductions will be gradual, starting at 4% of domestic flights on Friday, then rising to 5% on Saturday and 6% on Sunday, before hitting the full 10% next week.

The names of the affected airports - all high-traffic locations - would be released on Thursday, with reports indicating that some of the nation's busiest hubs, such as Hartsfield-Jackson Atlanta International and Chicago O'Hare International, will be included.

The cancellations could affect between 3,500 and 4,000 flights per day. International flights, however, will remain unaffected.

We are seeing pressures build in a way that we don't feel - if we allow it to go unchecked - will allow us to continue to tell the public that we operate the safest airline system in the world, Bedford noted.

Duffy insisted that air travel was still safe, and that the decision to cancel the flights was made to maintain safety and efficiency. If the shutdown continued and added more pressure to the system, additional restrictive measures may be required.

American Airlines stated it was awaiting further information from the FAA to determine which flights would be affected, while Southwest Airlines and Delta Air Lines continued to evaluate the situation.

So far, airports have begun to feel the strain, with reports of significant staff shortages leading to grounded flights shortly after the shutdown began.

Labor representatives have highlighted the serious impact on air traffic controllers, with many struggling to afford basic necessities amid the crisis.

This situation has prompted discussions regarding the safety of the air traffic system as workers balance their critical jobs with financial distress.