A computer glitch grounded United Airlines flights for the second time in six weeks early Wednesday, backing up thousands of passengers in the busy morning travel rush.

The glitch came shortly before trading was halted on the New York Stock Exchange as a result of technical problems.

While the timing of the two incidents raised concerns that a cyberattack was in progress, it appeared to be a coincidence.

There are "no signs of malicious activity at this time," a spokesman for the Department of Homeland Security told AFP, adding: "We're still investigating."

United flights around the country were prevented from taking off for more than an hour because "an issue with a router degraded network connectivity for various applications," the airline said.

"We fixed the router issue, which is enabling us to restore normal functions," United said in a statement.

About 900 flights had been affected by midday and more were expected to be delayed as United worked through the logistics, said Mark Duell, vice president of operations at FlightAware.com, which tracks flights and delays.

It will take hours, if not days, for the cascading delays to work their way through the system,

"There's not a lot of slack in the system," Duell told AFP.

Because United is so efficient when it comes to turning their planes around, any significant delay can cause major disruptions.

Flights get backed up on the tarmac because delayed planes are still at the gate, connections get missed and crew members have to be replaced.

A big issue, Deull said, "is if they lost an hour of productivity this morning, they can't structure the day so everyone just works until midnight because of rules on the number of hours of duty time they can have."

United apologized to customers "for any inconvenience" in a tweet posted shortly after flights were halted. It offered to waive change fees for anyone who wished to reschedule their flight rather than deal with the delays.

The grounding of all United flights due to "automation issues" was announced by the Federal Aviation Administration at 8:26 am EST (1226 GMT.) While United's smaller regional affiliates were soon cleared to fly, the carrier was unable to fix the problem for their entire service until 1347 GMT.

In early June, United flights were grounded in the United States for more than 40 minutes due to a computer problem. The airline also had a series of computer-related mass delays and cancellations following its merger with Continental in 2012.

Long lines at check-in and confused, frustrated customers were seen on local television stations Wednesday and customers took to social media to vent their frustration.

"I don't think 'network connectivity issue' does justice to what happened. 'Massive global computer failure" perhaps?'" Betsy Fischer Martin wrote on Twitter as she blogged her experience of being stuck on a plane with her teenaged daughter.

"My dogs left in crate 3+ hour outside because of delays! No walk or water offered! Still have flight ahead!," Haley Richardson, who was flying from San Diego to Newark, tweeted.

Others had a more humorous approach: "Have you tried turning the airplane off and then on again?" a person identified as @DGNHMR tweeted.

United, owned by United Continental Holdings, serves 373 destinations worldwide with a fleet of 691 mainline aircraft and 566 regional planes.

It operates nearly 5,000 flights a day and served 138 million passengers in 2014.

United shares were down 2.6 percent to $52.88 in afternoon trade.