South Korea's Defense Ministry says a marine corporal opened fire on colleagues at a frontline guard post Monday, killing three and wounding two.

A military spokesman said the corporal was on duty at the Yellow Sea island base of Ganghwa island, 70 kilometers west of Seoul.

The official said the dead included one officer and two enlisted men.

The spokesman said the suspect was detained and is undergoing questioning.

It is not clear what triggered the violence.