North Korea is building military camps for shore batteries on a western border island, Yonhap News Agency reported on Tuesday.
"Five bunker-shaped camps have been built on the island of Gal," an unidentified South Korean military officer was quoted as saying by Yonhap.
The North Korean island lies just above the de facto maritime border between the two Koreas, known as the Northern Limit Line.
"The North is expected to either deploy 122-mm multiple rocket launchers there or to use them as guard posts," the officer said, adding the military is closely monitoring the movements there.
The NLL was drawn by the U.S.-led United Nations Command at the end of the 1950-1953 Korean War. North Korea has never accepted it and draws a different border of its own to the south of it.
An area around the NLL has already been the scene of several deadly naval clashes between the two Koreas, which are still technically in a state of hostilities as their war ended in an armistice, not with a permanent peace treaty.
"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-mm ones," another military officer was quoted as saying.