North Korea appears to be building a military facility on a tensely guarded Yellow Sea border island, Seoul's defense ministry said Thursday.

"We've spotted that construction is under way on the small island of Gal," defense ministry spokesman Kim Min-seok said in a regular briefing, citing the North Korean island just above the Northern Limit Line.

"Considering the fact that the island is uninhabited and located near the border, we believe that chances are high for the North to be building a military facility," Kim said. "We are closely monitoring their moves with reconnaissance assets."

The island is located just 4.5 kilometers away from South Korea's Yeonpyeong Island, where the communist country launched an artillery attack in November 2010, killing four marines and civilians and wounding more than a dozen others.

"A handful of soldiers are digging in the ground, with no heavy equipment being employed." Seoul's Joint Chiefs of Staff officer said, requesting anonymity. "The construction could be either for a guard post or a military camp for shore batteries."

Drawn by the U.S.-led United Nations Command at the end of the 1950-53 Korean War, the NLL acts as the de facto sea border between the two Koreas. Pyongyang does not recognize the border, and the area has been the site of several bloody clashes between the two sides. (Yonhap)