South Korean troops fired "hundreds" of rounds at a man who was killed while trying to swim across the border to North Korea, a top army officer said Tuesday.

In the first official press briefing since the highly unusual incident on Monday, Brigadier General Cho Jong-Sul defended the border guards' actions, saying they had followed the correct protocol.

Nam Yong-Ho, a 47-year-old South Korean man, was fatally shot at around 2:30 pm (0530 GMT) while trying to swim across the Imjin river that makes up part of the western border with the North.

Cho said soldiers manning a nearby guard post had repeatedly shouted warnings at Nam to turn back, but he ignored them.

The commander of the unit then ordered his men to open fire, and 30 of them discharged their weapons.

"Several hundred shots were fired," Cho said.

Defections from South to North Korea are rare and there has been no case in the past 20 years of South Korean troops shooting anyone attempting the crossing.

Nam's precise motive is still unclear. Defence Ministry officials believe he was trying to defect, but have been unable to explain why he would seek to swim across the heavily guarded border in daylight.

He had clearly planned the crossing in advance and was wearing a home-made life preserver.

Cho insisted that the border guards had responded correctly.

"Soldiers are supposed to shoot those who ignore military warnings and run away at border areas," he told reporters.

"It was a very urgent situation considering he could have gone to the North fairly quickly using the flotation device he was wearing," he added.

Nam was carrying his passport, which showed he had sought political asylum in Japan in June, but had been rejected and deported.

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

Seoul's Vice Defence Minister also defended the soldiers' action, saying the heavily-fortified inter-Korean border was "different" from other frontiers.

"It may be hard to understand for foreigners... but the two Koreas are still at war," Baek Seung-Joo said at a briefing for foreign journalists in Seoul.

The minister said the military was trained to view any attempted defection as an "extreme situation" and assume a worst-case scenario -- a major national security breach -- should it be successful.

The 250-kilometre (155-mile) long Demilitarised Zone dividing the two neighbours is a depopulated no-man's land 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.