A computer outage at the US Federal Aviation Administration (FAA) brought flights to a standstill across America on Wednesday, with hundreds of delays quickly cascading through the system at airports nationwide.

The FAA ordered all US flights to delay departures until 9am Eastern Standard Time (10pm Hong Kong time), though airlines said they were aware of the situation and had already begun grounding flights.

At 8am Eastern Standard Time, there were more than 2,500 delayed flights within, into or out of the United States, according to the flight tracking website FlightAware, exceeding the number of all delayed flights on the previous day.

More than 150 have been cancelled.

Those numbers are likely to grow.

More than 21,000 flights were scheduled to take off in the US on Wednesday, mostly domestic trips, and about 1,840 international flights expected to fly to the US, according to aviation data firm Cirium.

The White House said that there is no evidence of a cyberattack, but President Joe Biden directed the Department of Transportation to investigate the cause of the disruption.

President Joe Biden addressed the FAA issue on Wednesday before leaving the White House. He said he had just been briefed by Transportation Secretary Pete Buttigieg, who told him they still had not identified what went wrong.

Buttigieg said in a tweet that he is in touch with the FAA and monitoring the situation.

Most delays were concentrated along the East Coast, but were beginning to spread west. Inbound international flights into Miami International Airport continued to land, but all departures have been delayed since 6.30am, said airport spokesman Greg Chin.

The FAA said it was working on restoring its Notice to Air Missions System.

“We are performing final validation checks and reloading the system now,” the FAA said. “Operations across the National Airspace System are affected.”

The agency said that some functions are beginning to come back on line, but that “National Airspace System operations remain limited.”

European flights into the US appeared to be largely unaffected. (AP)