Major US airports were operating normally on Tuesday evening after a glitch in the computer system for filing flight plans delayed hundreds of flights, 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 safely track planes in the air.

FAA spokeswoman Laura Brown said the problem was resolved around 6 pm EDT (2200 GMT), about 4 1/2 hours after a communications link failed in the system that processes flight plans at a facility south of Atlanta.

The agency's best guess is that 'hundreds' of flights across a wide swath of the United States from Dallas and Chicago to the East Coast had been delayed by the computer breakdown, Brown said, adding that the FAA would not have an exact count until Wednesday.

'There were some airports that were affected more than others,' she said. Airports in Boston, Chicago, Baltimore and Atlanta experienced the most delays as a result of the problem, she said.

The cause of the failure was not known but it was not due to a computer hacking attack, said Hank Krakowski, chief operations officer for the FAA's air traffic division.

'It appears to be an internal software processing problem. We're going to have to do some forensics on it,' he told reporters in a conference call.

Flight plans include information like the type of aircraft, destination and number of passengers.

The other flight-plan facility in Salt Lake City had to handle the entire country when the Atlanta system failed but the backup system quickly overloaded, Brown said.

'So what we had to do was dump all of the flight plan information that was in the system and then manually enter the people who were waiting to take off,' she said. 'That's what created the ripple effect throughout the system and created the delays that we had.'    

FAA spokeswoman Diane Spitalire said the agency had never experienced a computer problem this severe. 'We've had some equipment failures but not like this,' she 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.-Reuters