Flight departures in the US are slowly beginning to resume and a ground stop has been lifted after the Federal Aviation Administration  (FAA) scrambled to fix a system outage that grounded all departing aircraft.

The cause of the problem was unclear, but US officials said they had so far found no evidence of a cyber attack.

"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 FAA said in a tweet.

"The ground stop has been lifted.

"We continue to look into the cause of the initial problem."

Earlier the FAA had ordered airlines to pause all domestic departures after its pilot alerting system crashed.

More than 6,000 flights were delayed and nearly 1,000 cancelled according to the FlightAware website as officials said it will take hours to recover from the halt. The numbers were still rising.

The FAA is expected to implement a ground delay program in order to address the backlog of flights halted for hours.

Flights already in the air were allowed to continue to their destinations.

US President Joe Biden ordered the Transportation Department to investigate the outage and said the cause of the failure was unknown at this time.

Asked if a cyber attack was behind the outage, Mr Biden told reporters at the White House, "We don't know".

The FAA said it was working to restore a system that alerts pilots to hazards and changes to airport facilities and procedures that had stopped processing updated information.

Essential information system crashes

A NOTAM is a notice containing information essential to personnel concerned with flight operations, but not known far enough in advance to be publicised by other means.

Information can go up to 200 pages for long-haul international flights and may include items such as runway closures, bird hazard warnings and construction obstacles.

The system used to be telephone-based, with pilots calling dedicated flight service stations for the information, but has now moved online.

United Airlines said it had temporarily delayed all domestic flights and would issue an update when it learned more from the FAA.

Germany's Lufthansa and Air France both said they were continuing to operate flights to and from the United States, while the French airline said it was monitoring the situation.

The operator of Paris international airports — Paris Charles de Gaulle airport and Orly airport — said it expects delays to flights.

Latest headache for US travellers

This is just the latest headache for travellers in the US who faced flight cancellations over the holidays amid winter storms and a breakdown with staffing technology at Southwest Airlines.

They also ran into long lines, lost baggage, and cancellations and delays over the summer as travel demand roared back from the COVID-19 pandemic and ran into staffing cutbacks at airports and airlines in the US and Europe.

Julia Macpherson was on a United Airlines flight from Sydney to Los Angeles on Wednesday when she learned of possible delays.

"As I was up in the air I got news from my friend who was also travelling overseas that there was a power outage," said Ms Macpherson, who was returning to Florida from Hobart

Once she lands in Los Angeles, she still has a connection in Denver on her flight to Jacksonville.

She said there have been no announcements on the flight about the FAA issue.

Ms Macpherson said she had already experienced a delay in her travels because her original flight from Melbourne to San Francisco was cancelled and she re-booked a flight from Sydney to Los Angeles.

The FAA said that it would provide frequent updates as it made progress.

Reuters/AP