The Federal Aviation Administration ordered a nationwide ground stop on Friday afternoon, halting all departing flights across the United States until further notice.
"All departing traffic should remain groundstopped until further notice," the agency said, according to DW. The FAA cited "extraordinary circumstances and for reasons of safety."
ABC News reported that it was the second nationwide ground stop ordered since the Sept. 11, 2001 attacks.
Business Insider reported that the advisory, headed "National ground stop," followed an equipment outage at airports around the country. Chicago O'Hare, Chicago Midway and Newark in New Jersey were among the airports showing outages on the agency's status page, the outlet said. Business Insider said it had asked the FAA for comment and received no immediate reply.
Hindustan Times reported that the halt was linked to a rolling frequency outage.
AIRLIVE, an aviation news site, reported that no flights were being allowed to depart for US airports. Controllers were clearing the airspace as facilities moved toward "ATC Zero" status, an emergency condition in which a control facility cannot provide standard separation services. Aircraft already airborne were allowed to continue on their filed routes and land, AIRLIVE said.
Fox News carried the order as breaking news.
6abc Philadelphia reported that airports including Philadelphia International were affected.
The FAA had not published a cause or a timetable for restoring normal operations by Friday evening.