Investigations showed that the mines were planted by North Korean soldiers to target troops from the South, Maj. Gen. Koo Hong-mo said at a news briefing. They were typical land mines from the North that used wooden boxes, he said.

The mines were found just outside the South Korean guard post, which is 1,440 feet south of the military demarcation line, the official border that bisects the DMZ. They exploded as the soldiers opened the gate of a barbed-wire fence to begin a routine morning patrol, said Brig. Gen. Ahn Young-ho, a military investigator.

The explosions last week were the first of their kind in 48 years, South Korean officials said.

During and after the Korean War, both Koreas planted hundreds of thousands of mines in and near each side of the DMZ to thwart infiltrators.

Hundreds of South Korean villagers near the border have been injured or killed after stepping on or picking up stray mines.

The blasts last week took place near Paju, a front-line town north of Seoul. They were especially shocking because South Korean officials said that infiltrators from the North had come within striking distance of a South Korean guard post and had retreated, undetected, after planting three mines on both sides of a barbed-wire fence protecting the post.