The US Federal Aviation Administration (FAA) has lifted a grounding order on flights across America following a computer outage that resulted in thousands of delays at airports nationwide.

Earlier in the morning, the FAA ordered all US flights to delay departures until at least 9am EST (2pm GMT).

However, delays and cancellations continue to snowball.

Due to heavy congestion, the FAA cleared flights to depart at Newark Liberty and Atlanta Hartsfield-Jackson airports.

The agency said that normal air traffic operations are resuming gradually across the US following the outage.

More than 3,700 flights were delayed and more than 640 were cancelled earlier in the day.

The FAA said it is looking into the cause of the initial problem.

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 America, according to aviation data firm Cirium.

Some medical flights could get clearance, and the outage did not impact any military operations or mobility.

While the White House initially said that there is no evidence of a cyber attack, US President Joe Biden said “we don’t know” what caused the problem, and told reporters he has directed the department of transportation to investigate the issue.

Mr Biden said he had just been briefed by US transportation secretary Pete Buttigieg, who told him they had still not identified what went wrong.

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

“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.”

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

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

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

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.

Breakdowns in the NOTAM system appear to be rare.

European flights into the US appear to be largely unaffected.

Irish carrier Aer Lingus said services to the US continue, and Dublin Airport’s website showed that its flights to Newark, Boston, Chicago and Los Angeles were running on schedule.