SEOUL - A HAND grenade blast injured five soldiers on Sunday at a South Korean military post near the tense border with North Korea. Army officials were investigating, but said the incident had virtually no chance of being an attack by the North.

The grenade exploded on early Sunday at a guard post's barrack where 17 South Korean soldiers were sleeping, an army officer said on condition of anonymity, citing department policy.

The guard post is inside the 4-km-wide demilitarised zone bisecting the two Koreas, which are still technically at war.

Five soldiers were injured and one of them was in a critical condition, the army officer said.

The army dispatched a team to the site to investigate the incident.

'We're trying to find whether it was an accidental explosion or someone intentionally blew it up,' the officer said.

'But there is virtually no possibility that it was caused by a North Korean assault.'

In 2005, a South Korean soldier tossed a hand grenade and opened fire at a front-line army unit in a shooting rampage that left eight colleagues dead and several others injured.

Mr Kim Dong min later told investigators he went on the killing spree after being enraged by superiors who verbally assaulted him.

The incident raised serious questions about the level of discipline in South Korea's 680,000-member military.

All able-bodied South Korean men must serve about two years in the military under a conscription system aimed at deterring aggression from North Korea. -- AP