Transportation Secretary Sean Duffy warned there will be a 10% reduction in air travel capacity at 40 major airports in the US starting Friday morning, if the government shutdown continues.

The decision was made because air traffic controllers have been reporting issues with fatigue, the head of the Federal Aviation Administration (FAA) said at a briefing with Duffy on Wednesday.

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

During the shutdown, now the longest in US history, controllers have had to keep working without pay, prompting some to call out sick or take side jobs.

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, Reuters reported after the announcement, citing four unnamed sources.

The names of the affected airports - all high-traffic locations - will be released on Thursday, the officials said.

The cancellations could affect between 3,500 and 4,000 flights per day.

"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 said.

Duffy said air travel is still safe, and the decision to cancel the flights was being made to maintain safety and efficiency.

If the shutdown continues and adds more pressure to the system, additional restrictive measures may be required, Bedford said.

In a statement, American Airlines, the second-largest carrier in North America, said it was awaiting additional information from the FAA so it could determine which flights will be scrapped, but that "we expect the vast majority of our customers' travel will be unaffected".

A spokesperson for Southwest Airlines, the fourth largest, said in a statement that the company is still evaluating how the flight restrictions will affect its services, and will let customers know as soon as possible.

"We continue to urge Congress to immediately resolve its impasse and restore the National Airspace System to its full capacity," the spokesperson added.

Delta Airlines declined to comment. The BBC has also reached out to other major US airlines.

Once government funds ran out on 1 October, most federal workers were sent home and told they would be paid once the government reopened. Those deemed essential, like controllers, though, had to keep doing their jobs without pay.

Almost immediately after the shutdown started, airports began feeling the effects. Some had to ground flights for hours after air traffic controllers called out sick, while others relied on controllers from other airports.

Nick Daniels, the president of the labor union representing more than 20,000 aviation workers, put the situation into stark terms on Wednesday.

"Air traffic controllers are texting 'I don't even have enough money to put gas in my car to come to work,'" he told CNN.

"We base what we do day in and day out on predictability," he said. "Right now there is no predictability."

Duffy warned earlier this week that the flight cancellations may be coming, as half of the country's 30 major airports experience staff shortages.

He previously said there's a risk that comes with air traffic controllers taking on additional jobs during the shutdown, and had threatened to fire controllers who do not come to work.

"They have to make a decision, do I go to work and not get a paycheque and not put food on the table? Or do I drive for Uber or DoorDash or wait tables?" Duffy said on ABC on Sunday.