Hundreds of flights were cancelled across the United States on Friday after the Trump administration ordered reductions to ease strain on air traffic controllers who are working without pay amid congressional paralysis on funding the US budget.

Forty airports were due to slow down, including the giant hubs in Atlanta, Newark, Denver, Chicago, Houston and Los Angeles.

With Republicans and Democrats in a bitter stand-off over spending priorities, including over healthcare, Congress has ground to a standstill, leaving the entire federal funding spigot closed. Vast numbers of government employees, including vital airport staff, are working either without pay or are at home furloughed, waiting for the now nearly six-week crisis to end.

The impact remained unclear early on Friday, with the flight reductions due to take effect gradually, starting at four per cent and rising to 10 per cent next week if Congress still hasn’t reached a funding deal.

More than 800 flights scheduled for Friday were cancelled, according to tracking website FlightAware, which would be more than the previous three days combined.

Aviation analytics company Cirium said three per cent of US flights had been cancelled so far.