Thousands of US flights still cancelled or delayed two days after grounding of planes nationwide

Thousands of US flights were cancelled or delayed on Sunday as the US air-traffic control system continued to recover from a technical failure that led to a rare national ground stop.

Secretary of Transportation Sean Duffy said normal operations may not be restored until Monday.

"We have airplanes and crews that are out of position, so it takes a little bit of time," he told the BBC's US partner CBS. More than 5,000 flights were delayed and 2,000 cancelled on Sunday.

President Donald Trump urged Congress on Sunday to approve more money to modernise the air-traffic control system. The grounding of all aircraft scheduled to depart the US on Friday caused chaos at airports across the country.

More than 3,500 airplanes were in the air when the FAA announced at 15:54 EST (20:54 GMT) that flights would be grounded until further notice. Some were diverted to other airports and others flew on to their destinations.

In the aftermath, 8,500 flights were delayed and another 8,000 cancelled, according to FlightAware.

The stoppage left passengers stuck inside planes on the tarmac, unable to take off or to return to their gates. Others were stranded in airports, sleeping on the floor and desperately trying to get in touch with their airline. The ground stop also had a knock-on effect on schedules around the world, with some flights out of the UK and Europe cancelled.

The ground stop was lifted after about four and a half hours, but the impacts continued over the weekend.

On Saturday, airlines cancelled more than 5,900 flights and more than 7,000 others were delayed.

Investigators are still working to determine the root cause and who is at fault, but Duffy said the administration decided to ground flights after failures in a key system that allows air traffic controllers and pilots to communicate.

The transport secretary said a computer router installed as part of an upgrade from copper wire to fibre started sending out faulty signals.

He said on Sunday that the router - a piece of equipment which directs traffic across the network - was installed in El Dorado, Arkansas, and "flooded our network".

Duffy said that issues will continue to arise until the US makes major modernising changes to its air control system. The system needs a complete overhaul, he said.

"Let's not lie to each other," he said on NBC's Meet The Press. "We have to make investments into a network that has been neglected for decades."

In a post on Truth Social on Sunday, Trump urged US lawmakers to allocate the money, saying "we can get it built quickly and efficiently".

"It will be the BEST SYSTEM anywhere in the World. Much of our current air control system is 50 years old - RIDICULOUS! We have already started the work, but it should be done all at once, not in drips and drabs," he said.

Experts say the US uses antiquated technology for flight communications.

Aviation analyst Miles O'Brien described the US system as "a house that's about to be condemned".

He told the BBC: "I think most of us could look at this and go 'this is a system in the United States that we have allowed to decay'. It is worse than a fixer-upper."