This was all but frozen in 2001 after a collision between a Chinese fighter and an American spy-plane.

In an undated picture, a mock-up of the A-12 spy plane sits perched upside down on a testing pylon at Area 51-part of radar tests to reveal revealed how visible, or invisible, the design was to radar.

An American U-2 spy plane captured in the Soviet Union in 1960 had taken off from a secret airbase near Peshawar.