WASHINGTON, DC - The U.S. has begun allowing grounded flights across the country to resume. The Federal Aviation Administration (FAA) imposed a nationwide ban late Friday afternoon because of what were described as "safety concerns."

All departing flights were grounded as at 3:54pm (eastern time) Friday. Planes already in the air were permitted to land.

The ban lasted four hours before being lifted Friday night.

The FAA said the disruption had come as a result of a communications issue hitting air traffic control centers across the country.

"The FAA issued ​a nationwide ground stop for all U.S. airports following a communication issue ​impacting all Air Route Traffic Control Centers. FAA technicians are investigating the issue. We will provide immediate updates as we have them," Transportation Secretary Sean Duffy said late Friday, soon after the ban came into effect.

At 5:06pm (eastern time), the FAA posted on X: "There has been a temporary telecommunications outage - out of an abundance of caution all flights have been halted. Every flight has been accounted for and international flights will be handled on a case by case basis."

Shortly after 8:00pm Friday night, the grounding ended, and flights started taking off. "We started to have inconsistency in the data from the radars and our communication with aeroplanes, and we have procedures for this," Duffy said at a press conference after the ban had been lifted.

The transportation secretary said outdated air traffic control technology and routers overloaded with data caused the nationwide grounding. 

"What we have is an incredibly old air traffic control system. The president, for the last year, has been asking Congress to fully fund a full build of a brand new air traffic control system," Duffy explained.

"Our routers actually got overloaded with data, and they started to give us inconsistent data from those routers. And because of that, we shut the system down," he said.