South Korea's military has pledged to defend the Northern Limit Line(NLL), the de facto maritime border with North Korea amid brewing controversy over whether Pyongyang recognizes it as a sea border.

At a briefing Tuesday, spokesperson for the Joint Chiefs of Staff(JCS) Army Colonel Roh Jae-cheon said whatever the North claims, the NLL in the Yellow Sea is the actual maritime demarcation line that troops have defended with blood.

Concern over the North's recognition of NLL has been rekindled after the JCS told lawmakers during Friday's audit that Pyongyang had recently resumed its warning that South Korean ships illegally crossed its unilaterally-designated security line.

This comes after the leaders of the two Koreas agreed to turn the area into a "maritime peace zone," and President Moon Jae-in said North Korean leader Kim Jong-un recognized the NLL.

The JCS spokesperson then said while the North's latest warning message is considered a warning and not a threat, the South considers the fact that North Korean vessels haven't crossed the border as a sign that the regime acknowledges the NLL.