The US Federal Aviation Administration (FAA) is lifting 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).
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.
President Joe Biden has ordered an investigation into the FAA system outage that grounded flights.
Mr Biden told reporters at the White House that he had spoken to Transportation Secretary Pete Buttigieg and they should have a good sense in a couple of hours of what triggered the outage.
“We’ll respond at that time,” Biden said. Asked if the outage was caused by a cyberattack, he said, “We don’t know.”
“Aircraft can still land safely just not take off right now. We don’t know what the cause of it is.”
White House press secretary Karine Jean-Pierre said earlier in a Twitter post that there was no evidence of a cyber attack at this time.
Hundreds of US flights were grounded or delayed on Wednesday as the FAA scrambled to fix a system outage, with passengers told to check with airlines for updates.
Just before 7am on the eastern seaboard there were nearly 1,200 delayed flights within, into or out of the United States, according to the flight tracking website FlightAware.
Most delays were concentrated along the East Coast.
The FAA ordered all US flights to delay departures until 9am (2pm GMT), though airlines said they were aware of the situation and had already begun grounding flights.
The agency 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.”
In a statement, the FAA said some functions were beginning to come back online, and would give updates later.
Twitter users reported that flights were affected at airports including Philadelphia International Airport and Baltimore/Washington International Thurgood Marshall Airport.
The FAA’s Notice to Air Missions System (NOTAM) is an announcement that provides essential information to all personnel involved with flight operations, including pilots.
The system concerns the establishment, condition, or change of any facility, service, procedure or hazard in the National Airspace System (NAS), FAA said.
“NOTAMs have a unique language... to make communication more efficient,” FAA states on its website.
European flights into the US appeared 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.
“Aer Lingus plan to operate all transatlantic flights as scheduled today,” the carrier said in a prepared statement. “We will continue to monitor but we do not anticipate any disruption to our services arising from the technical issue in the United States.”