Normal air traffic operations are resuming gradually in the US after hours of delays, the Federal Aviation Administration (FAA) says.

It follows a problem with the system that alerts pilots to potential hazards on flight routes.

The FAA had said flights would take off again from 0900 ET (1400 GMT). It said it was continuing to look into the cause of the problem.

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

Flights from New Jersey's Newark airport, and Hartsfield-Jackson Atlanta International Airport resumed slightly ahead of others due to air traffic congestion in those areas.

Authorities are expected to impose a ground delay programme in order to address the backlog of flights which were halted for hours.

US President Joe Biden has been briefed, and the White House said there was no evidence of a cyberattack "at this point".

Speaking to reporters, the president said the FAA "expect [that] in a couple of hours they'll have a good sense of what caused it, and we'll respond at that time".

In a tweet, the White House Press Secretary said the president had called for a "full investigation".

The FAA said the source of the problem was its Notice to Air Missions System.

Major US airlines said they were closely monitoring the situation. American Airlines, which carries the most passengers annually in North America, said it was working with the FAA to minimise customer disruption.

Delta said it was "safely focused on managing our operation during this morning's FAA ground stop for all carriers", adding it would provide updates as soon as possible.

For international passengers, Air Canada - the foreign carrier with the most flights into the US - said the outage would impact on cross-border operations on Wednesday, but it couldn't initially say to what degree. The carrier said it would put in place a "goodwill policy" so affected passengers can change their travel plans.

Virgin Atlantic said it was continuing to operate its schedule of US flights departing from the UK. However, some US departures may be affected by delays.

British Airways said its flights to and from the US were continuing to operate as planned.

US Department of Transportation Secretary Pete Buttigieg said he had been in contact with the FAA on Wednesday morning, and that they were working to resolve the issue "swiftly and safely".

Passengers have posted on social media that they are experiencing delays.

"Always great to get up at 0400 to be at the gate in plenty of time for your flight that is now delayed by 37 minutes and counting," said Daniel Huard in a Facebook post. "This will be a long day of travel."

Michael Remy arrived to an airport in Virginia at 0600 ET (1100 GMT) planning to head to North Carolina for vacation. His flight was delayed right before boarding. "It is what is, so, you can only get so upset," he told the BBC. "I may have seen it differently if I was headed to a wedding or a funeral, though."