The attack by the 22-year-old soldier, identified only as Pte. 1st Class Kim, happened as many of his colleagues were asleep at an army unit stationed near the heavily militarized border in Yeoncheon, 70 kilometres north of the capital Seoul.

It wasn't clear what prompted the soldier to open fire on his comrades but South Korean military officials believe he did it out of frustration with military life or despair over his family environment, South Korea's Yonhap news agency said. The two injured soldiers were being treated at a military hospital.

The Defence Ministry said investigators were questioning Kim and refused to immediately give more details.