The Federal Aviation Administration halted departing flights nationwide on Friday after reporting communication problems affecting “multiple” air traffic control centers.

The issues were related to communications and radar problems, according to an internal message from Alaska Airlines, which was seen by CNBC.

“The FAA issued a nationwide ground stop for all U.S. airports following communication issues impacting multiple Air Route Traffic Control Centers,” the FAA said.

A ground stop halts flights at their departure point. Departing traffic was halted until further notice, the agency said, disrupting hundreds of flights. Disruptions tend to snowball if ground stops drag on as flight crews reach their maximum number of duty hours for a day.

Nationwide ground stops are extremely rare. In January 2023, the FAA halted flights because of issues with a pilot alert system. Before that, traffic was halted nationwide after the Sept. 11, 2001, terror attacks.

The agency said Friday afternoon that “FAA technicians are investigating the issue.”

Inbound international flights will be managed on a case by case basis, the FAA said.

United Airlines said in a statement that “airborne flights will continue.” United added that it “will work with our customers to get them to their destinations once this issue is resolved.”

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