The Federal Aviation Administration said Friday afternoon that it has grounded all flights across the United States due to “extraordinary circumstances” and issues related to safety.

The FAA’s website said the cause is an “equipment outage.” The site itself was down for a brief period at the time of publication, with a note that the website was “experiencing technical difficulties.”

Per an alert sent by the Air Traffic Control System Command Center, or ATCSCC, the FAA said the ground stop means that aircraft cannot take off from departure gates. The message said that the disruption was “due to extraordinary circumstances and for reasons of safety,” but did not specify further. The alert was not visible on the ATCSCC website at press time.

The alert stated that all departing flights must remain on the ground “until further notice.” Inbound international flights will be “managed on a case-by-case basis,” it said.

A spokesperson for the FAA told TechCrunch that it issued the ground stop “following communication issues impacting multiple Air Route Traffic Control Centers,” which manage the airspace across the United States.

Transportation Secretary Sean Duffy said in a post on X: “FAA technicians are investigating the issue.”

It’s not clear what caused the ongoing FAA equipment outage. The outage comes in the same week that the administration rolled out an AI-powered tool for air traffic controllers to save aircraft flight time and fuel costs — though it’s unclear if the ongoing outage is related.

Updated with comment from the FAA.