An extremely rare nationwide ground stop is bringing air travel across the U.S. to a halt after an apparent radio communications outage disrupted air traffic control operations.

The Federal Aviation Administration (FAA) issued the sweeping order Friday, directing all U.S. airports to stop departing flights until further notice.

The agency cited "extraordinary circumstances" and "reasons of safety" in an air traffic control advisory.

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U.S. airports report nearly 9,000 flight delays and more than 600 cancellations so far.

More than 11,000 flights are still scheduled to depart before the end of the day.

(MORE: Hail piles up, shuts down road in Philadelphia)

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Officials say FAA technicians are actively investigating the issue. It is unclear how long the ground stop will remain in effect.

During a Homeland Security press conference, officials said the President has been briefed on the situation and stressed there is no reason for the public to be afraid.

They described the problem as a telecommunications outage, not a cyberattack, adding that no planes currently in the air are without communications.

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"You just can't rule out anything at this point," said Homeland Security Secretary Markwayne Mullin. "We're going through that process, and we're going to find out what happened."

While no aircraft are taking off under the nationwide order, flights already in the air will be allowed to land. Inbound international flights will be handled on a case-by-case basis, according to the FAA.

The last nationwide ground stops of this scale occurred on Sept. 11, 2001, and in January 2023, when an FAA system used to distribute critical flight safety notices, known as NOTAM (Notice to Airmen), failed.

While the ground stop itself does not appear to be weather-related, Mother Nature could complicate efforts to rebook flights over the weekend.

The West coast is set to get rounds of rain this weekend, while parts of the Northeast are still cleaning up after a nor'easter. Travelers who manage to secure new flights could find themselves running into additional weather-related delays.

Meanwhile, the National Weather Service is clearing up confusion over reports of radar outages on their part:

(MORE: Storms could bring 50-foot waves to the West)

Another wrinkle: There are dozens of collegiate and professional sports teams currently traveling to new locations for tomorrow's games. If the ground stop continues, it could significantly impact weekend athletics.

For the latest on those games, check out our tailgate forecast.

If you are scheduled to fly Friday or over the weekend, check directly with your airline for the latest flight information before heading to the airport.

This is a developing story. Check back frequently for updates.