A South Korean soldier killed eight of his comrades after hurling a handgrenade among sleeping comrades and then opening fire on them, according to the South Korean Defence Ministry.
The private, identified by his surname "Kim," fired 40 shots from his rifle after throwing the grenade at his guard post in Yonchon, about 60 km north of the capital Seoul, a ministry spokesman said.
It was the highest number of deaths suffered by the South Korean army since 2000, Yonhap news agency said.
"Private Kim, who had suffered verbal violence and molesting from his senior, threw the grenade when he entered his barracks after overnight duty," Chang Suk-kyu, a chief spokesman for the ministry, told a briefing.
His senior, a corporal, was among the dead. Two soldiers were also wounded but were not in serious condition, Chang added.
Kim, 21, who was arrested after the shooting, joined the army in December 2004 and was deployed to an army unit on the frontier with North Korea in January, according to the army.
President Roh Moo-hyun expressed deep condolences to the families of the dead, while Defence Minister Yoon Kwang-ung said the armed forces would take steps to prevent future such incidents.