Passengers face lengthy delays as airlines get back up and running again following the US Federal Aviation Administration’s announcement that flights can resume.

As flights begin to resume across the US, airlines will be working to get some of the 21,464 flights scheduled to depart airports today up in the air, the BBC reports.

These flights, mostly domestic, have a capacity of nearly 2.9 million passengers, data from Cirium shows.

As of 14:30 GMT, almost 4,600 flights were delayed within, into or out of the US, flight tracking website FlightAware showed, without citing reasons.

Another 800 or so within, into or out of the country were cancelled.

Passengers at US airports have been reporting delays to their flights on social media.

One passenger at Logan Airport in Boston said on Twitter her flight was scheduled for 06:00 ET (11:00 GMT) but is still yet to take off.

Another said she had “already got to the airport too early for my liking” but was now suffering a three-hour delay.

A third passenger posted footage of himself and other passengers disembarking a flight from Orlando International Airport in Florida before take-off because of the delays.

He followed this up with a post an hour and a half later saying they were due to board their flight again after the FAA’s announcement that flights were resuming.

The major airlines operating in the US say they are “closely monitoring” the glitch that grounded millions of flights Wednesday.

American Airlines, which carries the most passengers annually in North America, says it’s working with the FAA to minimise customer disruption.

Delta says it “is safely focused on managing our operation during this morning’s FAA ground stop for all carriers”, adding it will provide more updates as soon as it can.

For international passengers, Virgin Atlantic says it continues to operate its schedule of US flights departing from the UK. However, some US departures may be affected by delays, it adds.

United Airlines says it has temporarily delayed all domestic flights and will share more when further information is made available by the FAA.

While British Airways says: “Our flights to and from the US are continuing to operate as planned.”

Speaking to reporters a little earlier, US President Joe Biden said the Federal Aviation Administration (FAA) doesn’t yet now what caused the issue but that he’ll be told when any new developments are made.

“They expect [that] in a couple of hours they’ll have a good sense of what caused it and we’ll respond at that time.”