The incident demonstrated the fragility of the ageing US air traffic control system, which Congress awarded US$12.5 billion for upgrades last year.

New Jersey Transit said a crew had been digging at one of its work sites 10 feet away from the utility markings and it was still trying to determine if Verizon or the contractor made the markings. 

Duffy initially blamed an Amtrak construction crew, which he said accidentally cut into a fibre line in New Jersey near New Brunswick, causing a telecom outage and forcing FAA to pause flights in the Northeast. Amtrak said it was not responsible.

Around 5.30pm (5.30am, Tuesday, Singapore time), New Jersey Transit said most of the fibre cables had been repaired and should be coming back online shortly. 

Newark Airport reopened for arriving flights after eight hours and LaGuardia also reopened after being shuttered for flights for much of the afternoon.

Verizon said it was working to repair the damaged cable near the Amtrak rail line and to restore connectivity as quickly as possible.

Flights at Philadelphia had again resumed late in the afternoon after a second ground stop was lifted. Reagan Washington National Airport also halted flights for more than an hour because of volume issues.

FAA Administrator Bryan Bedford said a replacement circuit had failed at Philadelphia Terminal Radar Approach Control, which was the primary communications for the centre. 

A ground stop at Boston, which stopped accepting flights because of spillover volume, was also lifted.

“The timing is absolutely horrible and I’m very sorry for the delays and cancellations,” Bedford told reporters. He stressed that safety was the top priority. 

“We’re going to do what is necessary to make sure the airspace is safe.”