US officials have said the scheduled capacity for flights would be cut by 10% from tomorrow in 40 busy air traffic areas of the country, with an ongoing government shutdown causing major staffing issues.
The announcement sent airlines scrambling to make significant reductions in flights in just 36 hours, while passengers inundated airline customer service hotlines with concerns about air travel in the coming days.
"There is going to be a 10% reduction in capacity at 40 of our locations," Transportation Secretary Sean Duffy told a news conference.
In a call with major US carriers, the Federal Aviation Authority (FAA) said capacity reductions at the airports would start at 4%, rising to 5% on Saturday and 6% on Sunday, before hitting 10% next week, industry sources told Reuters.
The FAA also plans to exempt international flights from the cuts.
Mr Duffy said the cuts could be reversed if Democrats agreed to reopen the government.
The shutdown, the longest in US history, has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration agents to work without pay.
The Trump administration has sought to ramp up pressure on the Democrats to end the shutdown and has increasingly raised the spectre of dramatic aviation disruptions to force them to vote to reopen the government.
Democrats contend Republicans are to blame for refusing to negotiate over key health care subsidies.
Tens of thousands of flights have been delayed since the shutdown began because of widespread air traffic control shortages.
Airlines say at least 3.2 million travellers have already been impacted by air traffic control shortages.
"We had a gut check of what is our job," Mr Duffy told reporters, citing a confidential safety assessment of the impact of the shutdown on controllers that raises concerns about their performance.
"Our job is to make sure we make the hard decisions to continue to keep the airspace safe," he said.
Reuters earlier reported the plan.
At a press conference, FAA Administrator Bryan Bedford said: "When we see pressures building in these 40 markets, we just can't ignore it."
"We can take action today to prevent things from deteriorating so the system is extremely safe today, will be extremely safe tomorrow," he added.
While the government did not name the 40 airports affected, the cuts were expected to hit the 30 busiest airports, including those serving New York City, Washington, DC, Chicago, Atlanta, Los Angeles and Dallas.
This would reduce as many as 1,800 flights and over 268,000 airline seats, according to aviation analytics firm Cirium.
Government officials said nothing would be final until the FAA published an order today.
The move is aimed at taking pressure off air traffic controllers.
The FAA is about 3,500 air traffic controllers short of targeted staffing levels and many had been working mandatory overtime and six-day weeks even before the shutdown.
The FAA also warned that it could add more flight restrictions after tomorrow, if further air traffic issues emerge.
United Airlines CEO Scott Kirby outlined the carrier's strategy for flight reductions, assuring staff and customers that long-haul international and hub-to-hub operations will remain unchanged.
The cuts will target regional flying and non-hub domestic routes instead.
Mr Kirby emphasised a flexible refund policy, telling employees that "any customer travelling during this period is eligible for a refund if they do not wish to fly - even if their flight isn't impacted".
American Airlines echoed a similar sentiment, indicating most of its customers would see minimal disruption.
Southwest, the largest domestic carrier, said it is evaluating how the cuts will affect its schedule and it will communicate with customers as soon as possible.
It urged politicians to immediately resolve the impasse over government funding.
The Association of Flight Attendants-CWA, which represents 55,000 flight attendants at 20 airlines, called the shutdown "cruel attacks on all Americans".
"The false narrative that this shutdown is a choice of either paying federal workers or protecting affordable healthcare is outrageous when both crises were manufactured by the exact people who can fix it," its President Sara Nelson said.
The federal government has mostly closed as Republicans and Democrats are locked in a standoff in Congress over a funding bill.
Democrats have insisted they would not approve a plan that does not extend health insurance subsidies, while Republicans have rejected that.
President Donald Trump and Republicans have been trying to intensify pressure on Democrats by increasing the pain felt by average Americans from the government shutdown.
The closure, which began 1 October, left many low-income Americans without food assistance, closed many government services and led to the furlough of about 750,000 federal employees.
Mr Duffy had warned on Tuesday that if the federal government shutdown continued another week, it could lead to "mass chaos" and force him to close some of the national airspace to air traffic.
Airlines have repeatedly urged an end to the shutdown, citing aviation safety risks.
Shares of major airlines, including United Airlines and American Airlines, fell about 1% in extended trading.
Airlines said the shutdown has not significantly affected their business but have warned bookings could drop if it drags on.
More than 2,100 flights were delayed yesterday.
On Tuesday, the FAA Administrator said that 20% to 40% of controllers at the agency's 30 largest airports were failing to show up for work.
The US Transportation Secretary said the authorities would also limit space launches to certain times of the day and are expected to impose restrictions on general aviation flights.
