[Photo Credit: Reuters]

The US Federal Aviation Administration lifted a nationwide ground stop on Friday ​after a communications failure at air traffic control centers caused chaos at airports and triggered the longest nationwide shutdown since the September 11, 2001, attacks.

Transportation Secretary Sean ‌Duffy said US airspace reopened at 8:15 p.m. Eastern Time (0115 GMT), but warned that departures would resume gradually and it would take time for airlines to restore normal operations.

More than 6,600 flights were canceled and 8,600 delayed, with another 1,500 already canceled for Sunday, according to FlightRadar24. Aviation analysts said the four-hour nationwide grounding could disrupt airline schedules for days.

Duffy said tens of thousands of ​passengers were stranded at airports around the country. Duffy said he and Homeland Security Secretary Markwayne Mullin had met with President Donald Trump in the Oval Office to discuss the ​outage.

The failure is the latest and most dramatic sign of strain on a US air traffic control system beset by aging technology, equipment and communications problems ⁠and a shortage of several thousand controllers.

Duffy said the trouble began at 3:36 p.m., when air traffic control centers experienced instability in radar and communications systems.

Duffy said the immediate problem had been fixed and that flying was safe, but warned that a broader overhaul of the aging air traffic control system was needed to prevent similar failures.

Duffy blamed Congress for failing to fund a new system that he said could prevent such outages, and called for $50 billion to modernize it.

Earlier on Friday, before the outage, the White House ​and Department of Transportation had urged Congress to approve $30.1 billion to upgrade the nation’s aging airports and air traffic control infrastructure.