South Korean forces are on high alert for possible provocations after a North Korean soldier was shot and wounded by his comrades as he defected across the demilitarised zone.

The rare defection occurred on Monday along the Joint Security Area (JSA) that is policed by both the South and North's militaries, with soldiers staring each other down face-to-face at the heavily fortified border that divides the Koreas.

Gunfire rang out and South Korean troops found the bleeding North Korean soldier - dressed in a combat uniform but unarmed - on their side. He was taken by helicopter to a nearby hospital for treatment.

The South's soldiers did not return fire.

The Korea Joongang Daily reported the North Korean was shot in the shoulder and elbow. His condition remains unknown.

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"In the afternoon today, a North Korean soldier defected from a guard post on the North Korean side of the JSA towards our side and our military took him," the Yonhap news agency quoted an unidentified military official as saying.

"The military has raised its alertness against the North Korean military's possible provocations and is maintaining its full readiness posture," the official added.

Yonhap described Monday's incident as "a very rare case" because North Korean troops stationed along the demilitarised zone are known for their loyalty to Pyongyang.

The 1950-53 Korean War - with the South backed by the United States and the North supported by China - ended in a ceasefire, not a peace treaty, meaning the conflict is technically still ongoing.

The US still has about 28,500 troops stationed in South Korea.

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Major military exercises involving three American aircraft carrier attack groups and seven South Korean warships off the tense peninsula will wrap up on Tuesday.

The massive military build-up led to an angry response from North Korea, which accused the alliance of planning a first attack against the communist country.