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

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 at least one officer and two enlisted men.

The spokesman said the suspect, who was wounded in the incident, has been detained.

It is not clear what triggered the violence.