The historian Gerald Haines writes that the air force, responding to purported UFO sightings during the Cold War years, frequently provided explanations that were untrue to deflect attention from the planes. "Over half UFO reports from the late 1950s through the 1960s were accounted for by manned reconnaissance flights (namely the U-2) over the US," Mr Haines wrote in the spring issue of Studies of Intelligence, an unclassified CIA journal. The article was found at the weekend on the Internet.
Concern about people finding out about the planes "led the air force to make misleading and deceptive statements to the public in order to allay public fears and to protect an extraordinarily sensitive national security project," Mr Haines wrote.
"While perhaps justified, this deception added fuel to the later conspiracy theories and the cover-up controversy" regarding the existence of UFOs, he added.
Mr Haines, a historian at the National Reconnaissance Office, based his article on a review of CIA documents from the late 1940s to 1990.
He described how the air force sought to deflect attention from development of its high-altitude experimental aircraft, the U-2 and SR-71.
Early U-2s were silver and reflected the sun's rays and often appeared as fiery objects to people below, Mr Haines said. They were later painted black. Air force investigators, "aware of the secret U-2 flights, tried to explain away such sightings by linking them to natural phenomena such as ice crystals and temperature inversions."
By 1956 the air force internally had clear explanations for 96 per cent of UFO sightings, Mr Haines wrote, referring to the experimental aircraft. "They were careful, however, not to reveal the true cause of the sighting to the public."
At the height of the Cold War the CIA hid its involvement in studies of UFO sightings because it feared that if word came out it would lead to a national hysteria that could be exploited by the Soviet Union.
John Pike, director of space policy at the Federation of American Scientists, said the study raises questions about other possible government cover- ups involving UFOs.
"The flying-saucer community is definitely on to something," he said.