The Defence Ministry said it believed three landmines exploded in the incident last Tuesday, hitting a patrol in the demilitarized zone -- a buffer zone stretching two kilometres 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.
Describing the attack as a "baseless act" and "wanton violation" of non-aggression accords, it urged the North to apologise for the attack and punish those responsible.
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The Defence Ministry declined to comment on what was meant by the term "harsh price" or to speculate on the options being considered for a response.
Analysts said the type of incident made a proportionate response difficult to gauge.
"Realistically, it's hard to see what South Korea can actually do," said Dan Pinkston, Korea expert at the International Crisis Group in Seoul.
There was no immediate response from North Korea to the charge that its military planted the devices.
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.
The rival Koreas remain technically at war because their 1950-53 Korean conflict ended in a ceasefire rather than a peace treaty.