"The FAA is making progress in restoring its Notice to Air Missions system following an overnight outage. Departures are resuming at @EWRairport [Newark Liberty International Airport] and @ATLairpor [Atlanta Airport] due to air traffic congestion in those areas. We expect departures to resume at other airports at 9 a.m. ET [14:00 GMT]," the FAA tweeted.
The number of US flights delayed due to the outage has topped 3,700, with over 600 flights canceled, according to FlightAware data.
Previously, the aviation administration ordered airlines to pause all domestic departures until 9 a.m. Eastern Time (14:00 GMT) following the failure of NOTAM.
"The FAA is working to restore its Notice to Air Missions System. We are performing final validation checks and reloading the system now. Operations across the National Airspace System are affected. We will provide frequent updates as we make progress," the US FAA said in the first update.
Meanwhile, the White House has said that there's no evidence of a cyberattack at this point, and President Joe Biden has ordered a full investigation into the system outage.
"There is no evidence of a cyberattack at this point, but the President directed DOT to conduct a full investigation into the causes," White House spokesperson Karine Jean-Pierre tweeted.
Secretary of Transportation Pete Buttigieg briefed President Biden on the situation, she added.
Biden, meanwhile, said that the cause remains unknown, according to the press pool.
"I just spoke with Buttigieg. They don't know what the cause is. But I was on the phone with him (garbled). I told them to report directly to me when they find out. Aircraft can still land safely, just not take off right now. They don't know what the cause of it is, they expect in a couple of hours they'll have a good sense of what caused it and will respond at that time," the president said.
Earlier in the day, media reported that the US FAA experienced a compute outage that affected NOTAM, which alerts pilots of potential flight route hazards. Roughly 1,200 flights within, into or out of the United States had been delayed just before 7 a.m. Eastern Time, according to the flight tracking website FlightAware. Tens of thousands of passengers are left stranded.