A North Korean soldier has defected after crossing the heavily-mined demilitarised zone border to South Korea.

The military in Seoul is questioning the soldier about the reasons he left, the South's Office of the Joint Chiefs of Staff said in a statement.

It comes amid high tension over the North's defiant pursuit of nuclear weapons.

This morning, South Korea announced they believe a drone sent from North Korea had been spying on a US missile defence system before it crash-landed on a hillside.

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

A South Korean soldier mans the border with North Korea, visible in the distance, near the sensitive Demilitarized Zone (DMZ) in Paju district, South Korea (file photo)

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).

A defence ministry official said: 'It has been confirmed that it took photos of the THAAD site in Seongju.'

He said they had 'high suspicions' the drone was from North Korea as it was similar to previous drones sent across the border by Pyongyang.

The device, which was flying at an altitude of between two and three kilometers, started photographing a few kilometers north of the South's southeastern county of Seongju.

It swept over the site before making a U-turn a few kilometers south of the site, the official said.

A small aircraft that South Korea's Military said is believed to be a North Korean drone