All flights in the United States are grounded until further notice “due to extraordinary circumstances and for reasons of safety,” the Federal Aviation Administration said in a statement.

The FAA issued the ground stop “following a communication issue impacting all Air Route Traffic Control Centers,” Secretary of Transportation Sean Duffy said in a social media post. “FAA technicians are investigating the issue.”

Air Route Traffic Control Centers are regional facilities that handle flights going across the country.

“Traffic should remain ground stopped until further notice,” the FAA said in an advisory. “International flights will be managed on a case by case basis.”