North Korea has installed several buoys near the tensely guarded western maritime border, the Defense Ministry said Thursday, apparently to control fishing boats.

"North Korea has set up around 10 buoys near the Northern Limit Line," Defense Ministry spokesman Kim Min-seok said in a regular briefing, referring to the de facto inter-Korean sea border in the Yellow Sea.

"We are analyzing the North's intention, and it appears to be out of necessity for its military," he said, without further elaboration. "We will defend the western sea border to the end."

Asked if the move could be interpreted as the North recognizing the NLL in substance, the spokesman said the communist country "has often acknowledged the NLL as the inter-Korean sea border out of necessity."

Pyongyang has denied the maritime border in the Yellow Sea, which was drawn by the U.S.-led United Nations Command at the end of the 1950-53 Korean War, and has demanded the line be drawn farther south.

Noting that the buoys are smaller than one meter and have been put just north of the South's islands of Baengnyeong and Yeonpyeong, a military source said they might have been placed to "better identify the sea border amid a lack of observation equipment."

A growing number of Chinese fishing boats illegally enter the waters, and North Korean patrol boats chasing after them have often crossed the NLL into the South, one of the major sources of inter-Korean tensions in the region.

This month alone, the North has violated the NLL four times, according to the Joint Chiefs of Staff.

"The buoys appear to be part of the North's plan to control fishing boats," another military source said, requesting anonymity.

"It also seems to be using them as a tool to check our defense posture near the border areas." (Yonhap)