Warning shots were fired by South Korean border guards on Thursday as North Korean soldiers approached the Demilitarized Zone after one of their comrades defected to the south, officials said.

Sounds of gunfire were reported from the North after South Korea's warning shots. South Korean defence officials could not immediately confirm the report.

Earlier, a North Korean soldier successfully defected via a portion of the heavily fortified border rarely visited by outsiders, officials in Seoul announced.

The "low ranking" soldier appeared at a checkpoint unharmed, according to South Korean media. His intentions are currently being assed by authorities.

North Korean soldiers have occasionally fled through the land border, but Thursday's defection came nearly 40 days after another North Korean solider crossed the jointly controlled area at the border amid a barrage of bullets fired by comrades.

He was shot multiple times and remains in hospital in South Korea.

The latest defection increased the number of North Korean soldiers who have fled through the land border this year to four, the Defence Ministry said. About 30,000 North Koreans have defected to South Korea, mostly via China, since the end of the 1950-53 Korean War.

Animosities run high on the Korean Peninsula as North Korea has been accelerating its weapons tests as part of its stated goal of achieving a nuclear missile capable of striking anywhere in the United States. Last month, North Korea test-fired its biggest and most powerful intercontinental ballistic missile.