By ANDY PASZTOR, SUSAN CAREY and JOSH MITCHELL - WALL STREET JOURNAL

A computer glitch that caused flight cancellations and delays across the U.S. Thursday has been resolved, the Federal Aviation Administration said, but it was unclear how long flights would be affected.

Major flight delays were reported in Washington, New York, Philadelphia and Chicago. Some flights were more than two hours behind schedule. Airports around the South also reported delays and cancellations.

The FAA said the computer problem, which lasted about four hours, was fixed around 9 a.m. EST. It started when a single circuit board in a piece of networking equipment at a computer center in Salt Lake City failed, the FAA said in a statement.

That failure prevented air traffic control computers in different parts of the country from talking to each other. Air-traffic controllers were forced to type in complicated flight plans themselves because they couldn’t be transferred automatically from computers in one region of the country to computers in another, slowing down the whole system.

Two large computer centers in Salt Lake City and near Atlanta were affected, as well as 21 regional radar centers around the country.

The problem produced a “domino effect” delaying flights around the country, the National Air Traffic Controllers Association said. Spokesman Doug Church said controllers were entering flight plans manually in some locations even after the glitch was fixed.

The FAA system is a major, cutting-edge program full of redundancies that are designed to keep it from going down. As such, the problem is quite different from localized radio communication or computer problems the FAA suffers from time to time.

The FAA said the problem started between 5:15 and 5:30 a.m. EST and affected mostly flight plans but also traffic management, such as ground stops and ground delays.

Flight delays began on the East Coast and rippled out to the west. The problem didn’t affect radar coverage or communications with planes in the air, the FAA said. The air-traffic controllers union, however, said the FAA systems that provide information on weather and wind speeds at airports weren’t functioning.

As of mid-morning, the delays appeared to wane. FlightStats.com, which captures FAA flight data, indicated that departures from Hartsfield-Jackson Atlanta International Airport were delayed by 20 to 60 minutes. But departures scheduled for later in the day were generally showing to be on time.

“The airport is working to minimize the impact on customers by bringing in additional staff and ensuring that all facilities are operational and fully maintained,” a statement from Hartsfield-Jackson officials said.

Departures from John F. Kennedy International Airport in New York were indicated running at more than an hour behind schedule for a handful of flights, according to FlightStats.com.

At Washington’s Dulles International Airport, flights destined for New York’s LaGuardia Airport averaged delays of one hour and 20 minutes. Traffic bound for Philadelphia International Airport from Dulles was running more than two hours behind. The FAA said gate hold and taxi delays at Dulles were averaging more than one and a half hours, and were lengthening.

US Airways Group Inc. said the FAA’s problems created a systemwide slowdown for its planes, with some being delayed up to 30 minutes. The airline said it brought additional staffers to its operations center in Pittsburgh to cope with the slower, manual processing of flight information.

Atlanta-based Delta Air Lines Inc., the largest U.S. airline by traffic, said it was experiencing some delays but continued to dispatch flights and was assessing a situation that remained fluid.

American Airlines, unit of AMR Corp. and the second-largest U.S. airline by traffic behind Delta, said the process of entering plans manually is working better, spokesman Tim Smith said. “It’s going much more smoothly now, so the system is flowing,” Mr. Smith said. “It took a little while to get them up to speed.”

American had yet to cancel any flights and was seeing delays of 10 to 20 minutes, though some stretched to an hour, Mr. Smith said. “We’ve had worse days driven by weather,” he said. “This is perhaps broader but not as extreme.”

An AirTran spokesman said as of 8 a.m. EST Thursday the airline had canceled 22 flight across the U.S. He said the cancellations were mainly at their Atlanta hub, but flights were affected nationwide.

Passengers are being asked to check the status of their flights online before going to airports.

—Dow Jones Newswires and the Associated Press contributed to this article.