South Korean soldiers have shot dead a man trying to swim across a border river into North Korea.

It is thought the South Korean man was trying to defect to the communist state.

A South Korean defence ministry spokesman said the man was carrying a South Korean passport that identified him as 47-year-old Nam Yong-Ho.

The spokesman said Nam approached the bank of the Imjin River which makes up part of the western border with the North.

He was spotted by soldiers manning a nearby guard post who fired warning shots and shouted at him to turn back.

Nam jumped into the river and an army corporal opened fire, killing him.

The spokesman said Nam was believed to have been trying to defect to the North and had jumped into the river with a flotation device to help him get across.

His passport indicated that he had been deported from Japan in June.

Defections from the South to North Korea are rare and there has been no incident in recent memory where South Korean troops have shot anyone attempting the crossing.

There was no immediate reaction from Pyongyang.

The incident came at a time of easing tensions between North and South Korea who were on a virtual war footing just a few months ago following the North's nuclear test in February.

Hours before the shooting, hundreds of South Korean factory supervisors drove across a nearby border crossing into North Korea after both sides agreed to reopen the Kaesong joint industrial zone shut down in April.

ABC/AFP