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

The FAA ordered all US flights to delay departures until 9am EST (2pm GMT), though airlines said they were aware of the situation and had already begun grounding flights.

At 8am EST (1pm GMT), there were more than 2500 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 flights have been cancelled, and 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 1840 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 cyber attack, but US President Joe Biden directed the Department of Transportation to investigate the cause of the disruption.

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

He said: “I just spoke to Buttigieg. They don’t know what the cause is. But I was on the phone with him about 10 minutes.

“I told him to report directly to me when they find out. Air traffic can still land safely, just not take off right now. We don’t know what the cause of it is.”

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 (11.30am GMT), said airport spokesman Greg Chin.

The FAA said it is working on restoring its Notice to Air Missions System (NOTAMs).

“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”.

United Airlines said that it had temporarily delayed all domestic flights and would issue an update once it learned more from the FAA. American Airlines said that it was closely monitoring the situation.

Before commencing a flight, pilots are required to consult NOTAMs, which list potential adverse impacts on flights, from runway construction to the potential for icing.

The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has now moved online.

There is a potential for widespread disruption because of the outage. All aircraft are required to route through the system, including commercial and military flights.

European flights into the US appeared to be largely unaffected.

The FAA said that it would provide frequent updates as it makes progress.