The Federal Aviation Authority (FAA) lifted the temporary halt placed on all domestic flights Wednesday after a system used to inform pilots of pre-flight safety notices experienced a massive outage.

The FAA said normal operations were "resuming gradually" after ordering a nationwide pause on all domestic departures until 9 a.m. on Wednesday.

"The ground stop has been lifted," officials said at about 8:50 a.m. ET. "We continue to look into the cause of the initial problem."

The affected Notice To all Air Missions (NOTAM) system is responsible for sending out flight hazards and real-time restrictions to pilots, including inclement weather and bird strikes. NOTAM's operation is vital for pilots before liftoff.

"The FAA is still working to fully restore the Notice to Air Missions system following an outage," said the agency, when it announced the temporary grounding of all planes nationwide. "The FAA has ordered airlines to pause all domestic departures until 9 a.m. Eastern Time to allow the agency to validate the integrity of flight and safety information."

It remains unclear if airlines will immediately resume services, as the interruption has engorged already existing congestion in the skies. Airlines for America, an association representing U.S. airlines, said the outage was "causing significant operational delays."

More than 4,500 flights within, into, or out of the United States had been delayed on Wednesday, according to FlightAware, a flight tracking service. Major U.S. carriers, including United Airlines, Delta, and American Airlines, said they had grounded flights in response to the situation.

Travelers were not the only ones keeping tabs on the status of the suspended airlines, as President Joe Biden also commented on the issue before the restoration of service.

"I just spoke with [Transportation Secretary Pete] Buttigieg," President Biden said, addressing the media on the South Lawn of the White House. "They don't know what the cause is. But I was on the phone with him about 10 minutes. I told him report directly to me when they find out.

"Aircrafts can still land safely, just not take off right now. We don't know what the cause of it is expected to be able to -- in a couple hours we'll have a good sense of what caused it. And we'll respond at that time."

The halt could not have come at the worst time for U.S. airlines as they continue to recover from the large-scale aviation meltdown over the Christmas holiday. Winter storms brought unseasonably cold temperatures to the majority of the country and caused travel chaos, with thousands of flights delayed or canceled.

Southwest Airlines canceled more than 15,000 flights over eight days after what it said was a breakdown in its scheduling systems. Other airlines are also dealing with issues related to canceled and delayed holiday flights.