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 standoff 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 Friday, with the flight reductions due to take effect gradually, starting at four percent and rising to 10 percent 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 percent of US flights had been cancelled so far.

The upheaval means ordinary Americans are now directly aware of the impacts from the fight in Washington, where the funding shutdown began October 1, increasing pressure on both parties.

"This isn't about politics, it's about assessing the data and alleviating building risk in the system," said US Transportation Secretary Sean Duffy, pushing back against criticism that the order aims to increase pressure on Democrats to end the shutdown. (AFP)