Three enlisted marines were killed Wednesday while standing guard at their base in the southeastern city of Pohang.
They were apparently victimized by what was believed to be an accidental collapse of a 38-year-old guard post.
A 22-year-old corporal, identified only by his surname Joo, and a 20-year-old private, identified only as Lee, were found dead under heavy debris from the two-story building, the Joint Chiefs of Staff said.
Another service member was taken to hospital for treatment, but was later pronounced dead, they said.
"The three were found under the ruins of the guard post, but the exact cause of their death has yet to be determined," Yonhap News quoted an official as saying.
An initial investigation has suggested the guard post, built in 1970, caved in as the old building could not support the weight of sandbags placed on its roof for protection against wind, according to officials.