Air traffic was delayed at several major US airports due to a glitch in the computer system for filing flight plans, the Federal Aviation Administration said.
The Department of Homeland Security said there was no link to terrorism and the FAA said the computer glitch did not affect its ability to track planes in the air.
An FAA official told reporters during a conference call that the agency hoped to have the problem resolved soon.
"It looks like we're slowly starting to dig out of this," said Hank Krakowski, chief operations officer for the FAA's air traffic division.
The problem began when a communications link failed in the system that processes flight plans at a facility south of Atlanta, FAA officials said. The cause of the failure was not known but it was not due to a computer hacking attack, Krakowski said.
"It appears to be an internal software processing problem. We're going to have to do some forensics on it," he said.
Flight plans include information like the type of aircraft, destination and number of passengers.
The other flight-plan facility in Salt Lake City was now handling the entire country, FAA spokeswoman Tammy Jones said. An FAA official in Chicago said workers there were filing flight plans by hand.
Airports in Boston, Baltimore, Charlotte, Atlanta and Chicago faced major delays, according to FAA and airport officials. Some delays were due to weather.
International flights were being given priority, the FAA said.
Flights in Washington, Miami, Cleveland and Houston were still taking off and landing normally, airport officials in those cities said.
American Airlines flights are facing delays in the Northeast "but not massively at this point," said Tim Smith, a spokesman for the airline's parent company, AMR Corp.
Similar computer failures have snarled traffic before but not to this extent, the FAA said.
An FAA communications outage in Memphis last year caused huge air-traffic snarls. The technicians' union blamed FAA cost-cutting for reducing backup standards.
