South Korea said Tuesday a drone believed sent from North Korea had been spying on a US missile defence system before it crashed.

The remains of the small drone, which was equipped with a camera, were retrieved by the South's military last week from a hillside where it had crash-landed close to the heavily-fortified inter-Korean border.

The military analysed the contents of the camera's 64-gigabyte memory chip and said it had been spying on the missile defence system known as the Terminal High Altitude Area Defense (THAAD).

"It has been confirmed that it took photos of the THAAD site in Seongju," a defence ministry official told journalists Tuesday.