South Korea's military fired 20 rounds of warning shots on Thursday as North Korean soldiers approached a borderline after their comrade defected to South Korea, officials said.

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

North Korean soldiers have occasionally - and often safely - fled through the land border.

Thursday's defection came nearly 40 days after another North Korean soldier crossed the jointly controlled area at the border, the only places where rival troops are only feet away, amid a barrage of bullets fired by colleagues.

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

On Thursday morning, another soldier, whose personal details were not immediately disclosed, used a different section of the border to defect.

When the soldier arrived at a front-line South Korean guard post, there was no shooting from the North, according to South Korea's Defence Ministry and Joint Chiefs of Staff.

But later on Thursday, South Korea's military detected North Korean soldiers approaching a military demarcation line at the border before it broadcast a warning and fired 20 rounds of warning shots from machine guns, said a South Korean defence official.

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.s