The Federal Aviation Administration issued a nationwide ground stop Friday afternoon, directing departing flights to remain on the ground until further notice, Axios reported, citing an air traffic control advisory.

The agency cited “extraordinary circumstances” and “reasons of safety.” Reuters reported that the nationwide hold followed serious air traffic control communications issues, including an earlier halt in Chicago. The FAA said international flights would be handled individually. The underlying cause of the communications problems was not established in that report.

This is a developing story. Travelers should check their airline for flight-specific updates and the FAA’s National Airspace System status page for advisories.