A computer failure at the regional air traffic control center at Leesburg yesterday morning caused flight delays of up to 15 minutes up and down the Eastern Seaboard until early afternoon, Federal Aviation Administration officials said.

National Airport and other air terminals delayed departures to assure that controllers, burdened with jobs normally done electronically by computer, would not be overwhelmed. Officials said safety was not compromised in the 160,000-square-mile area covered by the Leesburg facility.

Officials reported that three planes were a half hour late leaving National yesterday morning but said some of those delays apparently were routine and not caused by the computer failure. They were unable to put a number on total flights affected elsewhere. Traffic passing over the Washington area to and from Florida also was disrupted, FAA officials said.

Airlines gave contrasting accounts of the impact. An Eastern Airlines spokesman said there was none at all. United Airlines, reporting "very minimal" disruption, said typical delays were four to seven minutes, but one flight from Dulles International Airport to Los Angeles was held back almost half an hour.

Leesburg facility chief Angelo Viselli said the computer that processes the green radar images displayed on controllers' scopes failed for unknown reasons just before 8:30 a.m. The center then shifted to a back-up system that gives a less sophisticated account of planes' movements.

Electronic links that automatically transfer data about individual flights between Leesburg and other control facilities also broke down briefly during the morning.

Controllers at Leesburg and National and other airports used telephones to convey flight data and transfer control of planes from facility to facility, steps that are normally done by the computers electronically. Full computer service was restored at 12:40 p.m.

"We have had no report of any incidents or problems with the aircraft in the air," Viselli said. He said computer failures requiring use of the back-up for more than 20 or 30 minutes are extremely rare.

The back-up computers are simplified versions of the main ones. They do not, for instance, show the controller the flight number of a given "blip" on his screen, nor sound a "conflict alert" if two planes are on courses that may take them too close together, as the main system does.

The FAA has embarked on a multi-billion dollar program to modernize equipment in the air traffic control system.