South Korean forces fired warning shots on North Korean boats that crossed the border last week. North Korea doesn't recognize the de facto border and authorities from Seoul said they hadn't seen signs of provocation from Pyongyang.
South Korean Gen. Jung Seung-jo, chairman of the Joint Chiefs of Staff, was quoted by Yonhap News as saying South Korean forces were prepared to respond to any provocation from Pyongyang, however.
"Our military has drastically bolstered its capabilities and readiness to sternly punish enemy provocations with teeth-gritting determination" he was quoted as saying.
Tensions on the Korean Peninsula escalated last year following the death of North Korean leader Kim Jong Il. Various analysts have suggested Pyongyang this year may be preparing for a nuclear test.
Pyongyang said Seoul was creating tensions near the "ghost-like line" represented by the de facto maritime border in the Yellow Sea.
"Efforts to preserve the illegal 'northern limit line' will bring only death to them," a North Korean military spokesman was quoted as saying.