More than 5,000 US flights were cancelled or delayed on Friday, in the first day of new mandates forcing airlines to reduce air traffic during the government shutdown.

The new rules went into effect on Friday at 40 of the country's biggest airports to ease pressure on air traffic controllers and other federal workers reporting to work without pay amid the historic federal funding impasse.

Essential employees have been calling in sick or taking side jobs to make ends meet since the shutdown started last month.

To manage staffing shortages, the Federal Aviation Administration (FAA) issued an emergency order mandating a 4% cut to flights - which will rise to 10% by the end of next week.

The mandate - impacting major travel hubs in New York, Los Angeles, Chicago and Washington DC - came down as air traffic controllers had been reporting fatigue during staffing shortages as they work to ensure that US airspace remains safe for passengers, according to the FAA.

As essential workers, air traffic controllers are required to continue working without pay during the shutdown - which has become the longest ever in US history.

Continuing to work without pay for more than a month has led many to become ill with stress and forced others to take on second jobs to keep food on the table, unions for the employees have said.

Transportation Secretary Sean Duffy told the BBC on Friday that the flight cut mandate has not yet affected international travel due to international agreements that the US has to abide by.

But the chaos at airports could be just beginning. Duffy said in an interview with Fox News that cuts to flights could hit 20% if the government shutdown carries on and air traffic controllers continue to miss work.

Nick Daniels, the president of the National Air Traffic Controllers Association, said the controllers are being used as political pawns in the political standoff.