United Airlines briefly grounded all US flights because of a computer fault Wednesday morning, the second such setback for the carrier in less than six weeks.
“Automation issues” triggered the move, the Federal Aviation Administration said.
United said the glitch was an internal technology issue — not an outside threat. Spokeswoman Jennifer Dohm said a router problem reduced ‘‘network connectivity’’ for several software applications. ‘‘We fixed the router issue, which is enabling us to restore normal functions,’’ she said around midday.
A White House spokesman said President Obama was briefed on the problem and that it appeared unrelated to the outage later at the New York Stock Exchange.
United said more than 800 flights were delayed and about 60 were canceled. It said delays could linger throughout the day.
Flights at Logan Internationa Airport in Boston, where United is the fourth-largest carrier, were delayed as much as two hours between 8 a.m. and 10 a.m.
Some travelers took to social media to vent their frustration with the situation.
“I’m a little stressed, actually. I was in line for two-and-a-half hours,” said Don Dizon, an oncologist at Massachusetts General Hospital who was flying to Guam with four family members. He said the company wasn’t responding to passenger questions on social media. Late Wednesday, he said he was at the airport for nine hours and rebooked twice before his flight finally took off.
Lenka Safranova, who was preparing to fly to Vancouver from Logan Airport, said she was stuck in the United check-in line for about 20 minutes before the line started moving again. She was calm, though, because her flight was not scheduled to depart until about 1 p.m.
United, the second-biggest US airline, has suffered similar technology problems before, also leading to mass delays and cancellations.
On June 2, the airline briefly halted all takeoffs in the United States because of a problem in its dispatching system. At the time, United said that about 150 flights were affected.
United also struggled through a series of computer outages in 2012 after switching to the passenger-information system of Continental Airlines after the carriers merged. Those outages delayed hundreds of flights. High-paying business travelers were outraged; United CEO Jeff Smisek apologized for failing to provide good service.
After a 2010 merger, United elected to combine many computer systems and frequent-flier programs all at once. Executives believed that any disruptions would be short-lived. In contrast, Delta and Northwest integrated their systems in stages after a 2008 merger, and American Airlines is taking Delta’s same go-slow approach now as it absorbs US Airways.
Other airlines, have been hit by computer problems, too, however. In April, more than 50 American flights were delayed when a software glitch prevented pilots from seeing some airport maps on their tablet computers, for instance.
After Wednesday’s problems, United apologized and said customers could change travel plans without paying the usual $200 fee. In some cases, the airline said it would also waive any difference in fare for the rescheduled trip.
‘‘We don’t know everything behind this morning’s issues yet, but today’s incident underscores the sense that something is very wrong at United,’’ said Gary Leff, cofounder of the frequent-flier website MilePoint.
‘‘How could a router bring down one of the world’s largest airlines?’’ asked Henry Harteveldt, a travel analyst at Atmosphere Research Group. he said it appeared that United lacked enough redundancy in its technology systems.
Still, Harteveldt said he doubted that United would lose many business-travel customers because of technology hiccups that could happen to any carrier.
Globe correspondents Jack Newsham and Jessica Geller contributed to this report.
Material from Bloomberg News was used in this report.