Naval ships opened fire after North Korean patrol vessel crossed disputed sea border in the Yellow Sea, officials said.
South Korea has accused North Korea of planting landmines that maimed two soldiers on border patrol, sending military tensions on the Korean peninsula soaring as it threatened to make Pyongyang pay a “harsh price”.
The defence ministry said on Monday that it believed three landmines exploded in the incident last Tuesday, hitting a patrol in the demilitarised zone (DMZ) – a buffer zone stretching 2km 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.”
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, a 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.
The United Nations Command that monitors the ceasefire accord said on Monday that it had conducted a special investigation into last week’s blasts and concluded they were North Korean “wooden box” land mines placed on a known South Korean border patrol path.
“The investigation determined that the devices were recently emplaced, and ruled out the possibility that these were legacy landmines which had drifted from their original placements,” it said in a statement.
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.