Normal air traffic operations are resuming gradually across the US on Wednesday following a technical failure of a key pilot notification system that resulted in the grounding of thousands of flights for several hours.

The Federal Aviation Administration announced earlier in the morning that it suffered an outage of its Notice to Air Missions or NOTAMs, which alerts pilots and other personnel about airborne issues.

The FAA ordered airlines to delay all domestic departures early Wednesday morning, but lifted the ground stop just before 9 a.m. Eastern after several hours.

"Normal air traffic operations are resuming gradually across the US following an overnight outage to the Notice to Air Missions system that provides safety info to flight crews. The ground stop has been lifted. We continue to look into the cause of the initial problem," FAA said in a tweet.

According to FlightAware, a flight tracking company, more than 3,700 flights within, into or out of the US were delayed and more than 600 were cancelled due to the system failure.

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, AP reported, citing aviation data firm Cirium.

The FAA earlier said it is making progress in restoring its Notice to Air Missions system and departures are resuming at the Newark Liberty International Airport and Atlanta International Airport due to air traffic congestion in those areas.

"We expect departures to resume at other airports at 9 a.m. ET," it said.

"All flights currently in the sky are safe to land. Pilots check the NOTAM system before they fly. A Notice to Air Missions alerts pilots about closed runways, equipment outages, and other potential hazards along a flight route or at a location that could affect the flight," FAA said.

It earlier said that operations across the National Airspace System are affected.

President Joe Biden was briefed by Secretary of Transportation Pete Buttigieg on the FAA 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. The FAA will provide regular updates," White House Press Secretary Karine Jean-Pierre said in a tweet.

Transportation Secretary Buttigieg said that he is in touch with the FAA and monitoring the situation.

"I have been in touch with FAA this morning about an outage affecting a key system for providing safety information to pilots. FAA is working to resolve this issue swiftly and safely so that air traffic can resume normal operations, and will continue to provide updates," he said in a tweet.