A newly declassified CIA document confirms the existence of famed "Area 51" in Nevada, but conspiracy theorists will be disappointed the spy agency offers no proof of alien spaceship landings at the site.

"Area 51" has long been fodder for wild UFO tales claiming the US government imposed secrecy over the site northwest of Las Vegas to cover up evidence of extraterrestrials.

Instead of encounters with flying saucers, the documents released by the Central Intelligence Agency on Thursday recount the history of "Area 51" as a testing range for the government's U-2 spy plane during the Cold War.

The reconnaissance aircraft was designed to spy on the Soviet Union at high-altitude, and its development was shrouded in secrecy.

In April 1955, the CIA chose a remote dry lakebed in the Nevada desert as a testing ground, which was designated on maps as "Area 51."

But test flights for the U-2 aircraft were at a much higher altitude than commercial airliners or other military planes. In the 1950s, commercial planes flew at between 10,000 and 20,000 feet and warplanes such as B-47 reached altitudes of less than 40,000 feet.

The U-2 planes flew at above 60,000 feet, and reports of unidentified flying objects in the Nevada desert started to roll in, the report said.

"High altitude testing of the U-2 soon led to an unexpected side effect -- a tremendous increase in reports of unidentified flying objects (UFOs)," it said.

The reports of UFOs often came from pilots from commercial airliners in the early evening hours, with the U-2 plane's silver wings reflecting the rays of the sun high in the sky.

The surveillance planes appeared to be "fiery objects" at an altitude believed to be out of reach for manned flight, it said.

Anxious to avoid exposing the ultra-secret U-2 program, Air Force officers explained the sightings as merely due to natural phenomena.

U-2 and other surveillance flights "accounted for more than one-half of all UFO reports during the late 1950s and most of the 1960s," it said.

The 400-page report, titled "Central Intelligence Agency and Overhead reconnaissance: The U-2 and Oxcart Programs, 1954-1974," was released as a result of a Freedom of Information request by the National Security Archives at George Washington University.

Area 51's location has been an open secret for years but government documents released previously had not acknowledged its existence and role in such a detailed way.