NEW YORK -- United Airlines grounded flights across the country for part of Wednesday after experiencing computer problems.
An airline spokesman said a computer router problem reduced "network connectivity" for several software applications.
Around midday, spokesman Jennifer Dohm said, "We fixed the router issue, which is enabling us to restore normal functions."
The Federal Aviation Administration lifted a ground-stop order after nearly two hours, allowing United planes to fly again.
United did not immediately say how many flights were affected. The airline has suffered similar technology problems before, also leading to delays and cancellations.
A spokesman for Bill and Hillary Clinton National Airport/Adams Field said United Airlines service resumed in Little Rock about 8:30 a.m. and that the delay had little effect on passengers leaving the airport.
The nation's second-biggest airline briefly halted all takeoffs in the U.S. on June 2 because of a problem in its flight-dispatching system. United said then that about 150 flights were affected.
United also struggled through a series of computer failures in 2012 after switching to the passenger-information system of Continental Airlines after that carrier merged with United. Those glitches caused hundreds of flights to be delayed. The delays angered high-paying business travelers, and United Chief Executive Officer Jeff Smisek apologized for failing to provide good customer service.
After the 2010 merger, United decided to combine many computer systems and frequent-flier programs at once. Executives believed that any disruptions would be short-lived. By contrast, Delta Air Lines and Northwest Airlines integrated their systems in stages after a 2008 merger, and American Airlines is taking Delta's approach now as it absorbs US Airways.
Other airlines, however, have also been hit by computer problems. In April, more than 50 American flights were delayed when a software glitch prevented pilots from seeing some airport maps on their tablet computers.
After Wednesday's problems, United apologized to customers and said they could change travel plans without being charged the usual $200 reservation-change 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, co-founder of frequent-flier website MilePoint.
Shares of Chicago-based United Continental Holdings Inc. closed Wednesday down $1.49, or 2.7 percent, to $52.82.
Mike Adler and his wife arrived at Boston's Logan International Airport on Wednesday morning, ready to begin their vacation to Los Angeles. What they found were long, snaking lines and frustrated travelers at the United counters.
Adler, 41, said he soon learned that the airline was having technical problems -- flight attendants were writing luggage tags by hand and converting printed boarding passes into scribbled pieces of paper.
"It's frustrating on the front end of your vacation to get to an airport and see incredibly long lines," he said. "It's one thing when weather causes your plane to be delayed, when it's things they don't have control over ... but it's an entirely different thing for their entire system to go down.
"It just weakens your trust in whether they really can run an airline of this size," he said.
United already lags in popularity compared with airlines such as Delta, and Wednesday's bad publicity might not help, said Seth Kaplan, managing partner for the publication Airline Weekly.
"United has not established that kind of reputation as being an extraordinarily reliable airline," he said. "Even though it has overall improved in recent years, these very visible incidents are going to do nothing to help that reputation."
But, he added, it was possible for the airline to recover from the incidents.
"I am more frustrated than anything," said Jay Mitchell, an executive-search consultant who was booked to fly to Houston from Fort Lauderdale, Fla. A friend alerted him that United's planes had been ordered parked, and he abandoned his trip to the airport.
"Almost half" of his United flights in the past two or three months have been delayed to some extent, according to Mitchell, who said he flies almost weekly. "It's gotten pretty consistent to where I now have to plan a buffer around my travel time."
Information for this article was contributed by Scott Mayerowitz, David Koenig, Michelle Chapman, Matt Small and Joan Lowy of The Associated Press; by Samantha Masunaga, Christine Mai-Duc and Taylor Goldenstein of the Los Angeles Times; and by Michael Sasso and Lauren Thomas of Bloomberg News.