The border with North Korea is being closely monitored.
South Korean troops are engaged in a shootout with a soldier who shot dead five colleagues and fled his post near North Korea's border, reports say.
The conscript soldier exchanged gunfire with troops seeking to capture him near a school in the border town of Goseong, Gangwon province, Yonhap reported.
It is not clear why the sergeant, identified only by his surname, Lim, opened fire on Saturday.
There have been similar shooting cases in the South Korean army in the past.
Sgt Lim shot fellow members of his 22nd infantry division in the remote guard post and then escaped with his rifle and some ammunition, army officials said.
Seven other troops were injured in the incident, but are expected to survive.
Thousands of troops were deployed to block possible escape routes, guard the border between North and South Korea, and protect areas where civilians live, officials said.
Guarding the demilitarised zone, which divides the two Koreas, is considered one of the toughest jobs for military conscripts, the BBC's Kevin Kim in Seoul reports.
Sgt Lim "threw a grenade and then opened fire" after completing his day shift, Yonhap reported.
He was armed with a semi-automatic K2 rifle, which is similar to an M16 and is one of the standard South Korean army rifles, officials said.
An officer said Sgt Lim "was on the list of those who require special attention, as he had difficulties in adapting to the military life," the South Korean news agency reported, adding that he had been due to be discharged in three months.
The defence ministry spokesman said the incident was "regrettable", adding: "We truly apologise for causing concern to the nation."
There have been similar incidents in South Korea's military, which operates under a conscription system. Bullying and mental problems have been blamed for previous attacks.
In 2011, a marine opened fire on his colleagues and tried to blow himself up with a grenade. Four soldiers were killed in the attack.
Tens of thousands of soldiers from both North and South Korea are stationed along their joint border, one of the most heavily fortified in the world.
The two Koreas were divided at the end of the 1950-53 Korean War, which ended in an armistice rather than a peace treaty. As a result, they remain technically at war.