South Korean troops shot dead a man trying to swim across a border river into North Korea on Monday after he ignored repeated warnings to turn back, the defence ministry said.

A mistry spokesman said he was carrying a South Korean passport that identified him as Nam Yong-Ho, 47.

According to the spokesman, Nam had approached the bank of the Imjin river that makes up part of the western border with North Korea at around 2:30pm (0530 GMT).

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

"He jumped into the Imjin river, ignoring repeated warnings to stop," the spokesman told AFP. "The soldiers opened fire and his body has been retrieved."

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 South to North Korea are very 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 a joint industrial zone shut down in April.

Because the 1950-53 Korean War concluded with an armistice rather than a peace treaty, North and South Korea technically remain at war.

Four kilometres wide and 248 kilometres long, the Demilitarised Zone dividing the two neighbours is a depopulated no-man's land of heavily-fortified fences bristling with landmines and listening posts.

While the number of confirmed defections from South to North has been tiny, more than 23,500 North Koreans have escaped the other way since the end of the war.

But virtually all avoid any attempt to cross the land border, with most escaping to China and then to a third country where they request resettlement in the South.