North Korea is building military camps for shore batteries on a tensely guarded Yellow Sea border island, the South's military authorities said Tuesday.

"Five bunker-shaped camps have been built on the island of Gal," a military officer said, requesting anonymity, citing the North Korean island just above the de-facto inter-Korean western sea border of the Northern Limit Line.

"The North is expected to either deploy 122-millimeter multiple rocket launchers there or to use them as guard posts," he said, adding the military is closely monitoring the movements there. 

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 two Marines and two civilians, and wounding more than a dozen others.

If completed, the facilities will further heighten tensions in the areas, which have seen bloody inter-Korean naval clashes over the decades.

"Our military has positioned Spike missiles on the northwestern islands in the Yellow Sea, which are capable of precisely striking the North's coastal artillery pieces, including the 122-millimeter ones," another military officer said.

Pyongyang does not recognize the NLL and has demanded the line be drawn farther south. After the 1950-53 Korean War ended in a ceasefire, the U.S.-led United Nations Command drew the border. (Yonhap)