NEW YORK (AP) — A computer outage at the Federal Aviation Administration brought flights to a standstill across the U.S. early Wednesday, with thousands of delays quickly cascading through the system at airports nationwide.

The FAA ordered all U.S. flights to delay departures until at least 9 a.m. Eastern. Due to heavy congestion, the FAA cleared flights to depart at Newark Liberty and Atlanta Hartsfield-Jackson airports and departures were expected to resume at other airports soon.

There were more than 3,700 flights delayed by 8:30 a.m. Eastern, more than all the delayed flights for the entirety of the previous day, according to the flight tracking website FlightAware. More than 640 have been cancelled. That number is expected to grow.

Those numbers are likely to grow, and the groundings impact almost all aircraft, including shipping and passenger flights. More than 21,000 flights were scheduled to take off in the U.S. today, mostly domestic trips, and about 1,840 international flights expected to fly to the U.S., according to aviation data firm Cirium.

Some medical flights can get clearance and the outage is not impacting any military operations or mobility.

Early Wednesday, flights for the U.S. military’s Air Mobility Command had not been impacted, said Air Force Col.

Damien Pickart, a spokesman for Air Mobility Command is responsible for all the troop movement and supply flights, such as the C-17s that carry the president’s motorcade vehicles when he travels, but also all the flights that transport troops from one base to another. Air Mobility Command was working with the FAA on the issue.