The US government has for the first time acknowledged the existence of a military base in Nevada known as Area 51, according to new declassified documents.

The section has been associated with the search for extraterrestrial life and the government conspiracy to cover up the existence of aliens, however, the documents suggest that it was merely a testing site for the government's U-2 and OXCART surveillance programmes.

Former Governor of Nevada Bob Miller unveils a road sign near Area 51, in April 1996. Credit: Reuters/Steve Marcus

The U-2 programme conducted surveillance around the world, including over the Soviet Union during the Cold War.

George Washington University's National Security Archive obtained a CIA history of the U-2 spy plane programme through a public records request in 2005. The original request in 2002 had redacted all mentions of Area 51, but has been restored in the latest version.

US government acknowledges the existence of 'Area 51'. Credit: National Security Archive

National Security Archive senior fellow Jeffrey Richelson believes the new document shows the CIA is becoming less secretive about Area 51's existence, if not about what goes on there.

Officials have already acknowledged in passing the existence of the facility in central Nevada where the government is believed to test intelligence tools and weapons.

UFO expert Timothy Good said some secrets still remain despite the declassification.