SEOUL: South Korea Thursday repeated demands that North Korea apologise for opening dam floodgates and killing six Southerners, as officials checked whether its action violates international rules.

The North on Sunday discharged millions of tons of water from a dam into a cross-border river and six South Korean campers downstream were swept away.

The incident has stirred anger in the South and threatens to damage relations which had lately been improving after more than a year of hostility.

"The government maintains North Korea's responsible authorities should give a full account and an apology for this matter," said unification ministry spokesman Chun Hae-Sung.

The spokesman said relevant ministries are investigating whether the unannounced release of the dam water violates international laws.

Seoul said 40 million cubic metres (52 million cubic yards) was released from the Hwanggang Dam into the Imjin river, which flows down into South Korea before reaching the Yellow Sea.

The North has yet to respond to the call for an apology. On Monday it blamed a sudden surge in the dam's water level for the "emergency" release and promised to give prior warning of future discharges.

The response did not pacify Seoul. Officials said there had been no heavy rain in the North in recent days to explain the need for a release of water.

Unification Minister Hyun In-Taek told parliament Wednesday the discharge of the dam water was intentional but the motive was still being studied.

His ministry said Hyun was not necessarily saying the North released the water as a deliberate act of aggression.

Seoul has raised fears in the past of a "water attack" from Pyongyang.

Last month the North made several peace overtures. It freed five South Korean detainees, eased border curbs, sent envoys for talks with President Lee Myung-Bak and proposed a new round of family reunions.

The unification ministry said it hopes the reunions, of relatives divided since the 1950-1953 war, would go ahead late this month despite the flood incident.