South Korea on Monday accused North Korea of planting landmines that maimed two soldiers on border patrol, ramping up military tensions as it threatened to make Pyongyang pay a "harsh price".

South Korea on Monday accused North Korea of planting landmines that maimed two soldiers on border patrol, ramping up military tensions as it threatened to make Pyongyang pay a "harsh price".

The Defense Ministry said it believed three landmines exploded in the incident last Tuesday, hitting a patrol in the demilitarized zone (DMZ) -- a buffer zone stretching two kilometers on either side of the actual frontier line dividing the two Koreas.

"We are certain they were North Korean landmines planted with an intention to kill by our enemies who sneaked across the military border," ministry spokesman Kim Min-Seok told reporters.

One soldier underwent a double leg amputation, while the other had one leg removed.

In a statement, the South's Joint Chiefs of Staff said its military would make North Korea "pay a harsh price proportionate for the provocation it made."

Describing the attack as a "baseless act" and "wanton violation" of non-aggression accords, the statement urged the North to apologize for the attack and punish those responsible.

The Defense Ministry declined to comment on what was meant by the term "harsh price" or to speculate on the options being considered for a response.

The last direct attack on the South was in November 2010 when North Korea shelled the South Korean border island of Yeonpyeong, killing two civilians and two soldiers.

South Korea responded by shelling North Korean positions, triggering brief fears of a full scale conflict.

More than a million mines are believed to have been planted along the inter-Korean border, including those which were air-dropped in great numbers in the 1960s at the height of a Cold War confrontation with the North.