SEOUL, Korea, Republic Of -- A South Korean soldier shot his fellow reservists at a mandatory training session in Seoul on Wednesday, killing two and injuring two others before killing himself, officials said.

The 24-year-old soldier, surnamed Choe, abruptly turned around after firing one round during shooting training and fired seven rounds with a K-2 rifle at his fellow reservists, army and defence officials said on condition of anonymity because of office rules. He then used the ninth of the 10 bullets he'd been given to kill himself, they said.

One of the army officials said Choe shot himself in the forehead and the other two who died were shot in their heads, although he couldn't be more specific about the locations.

It was not immediately known why Choe opened fire. Yonhap news agency, citing the army, reported that Choe was put in a group of soldiers who needed special attention when he was on active service and that he received medical treatment for depression. The Defence Ministry and the army couldn't immediately confirm the report.

In an odd twist, South Korean pop star PSY, who is also a reservist, received about an hour of training at the site before leaving about 20 minutes before the shooting occurred because of personal business, according to an official from the rapper's agency YG Entertainment. The official, who refused to be named, citing office rules, said the singer was allowed to leave after moving his remaining training hours to another date.

Shootings by soldiers at South Korean military barracks have happened with some frequency in recent years, raising concerns about bullying and mental health conditions in the country's armed forces. But a shooting spree involving a reservist is unusual.

Last year, a South Korean soldier threw a grenade and opened fire on colleagues, killing five and wounding seven others. He later told investigators that he shot fellow soldiers after seeing a drawing they made of him that he considered insulting.

All able-bodied South Korean men must serve about two years of compulsory military service under a conscription system aimed at handling threats from rival North Korea. Past rampages raised questions about the discipline and readiness of South Korea's military.

After finishing their military duties, South Korean men undergo annual training as members of the reserve force for eight years and their training involves shooting drills for the first six years, according to the Military Manpower Administration.