South Korea protested to North Korea Monday after a wall of water was suddenly released from a dam in the North, creating a flash flood that swept away six people south of the border.

The floodwaters hit five campers and a fisherman early Sunday after water was released from the dam into the Imjin River that crosses the frontier, briefly swelling it to twice its normal depth.

"The government has expressed regret that a North Korean dam along the Imjin River released water yesterday without prior notice and caused big damage, including six of our citizens going missing," the South's unification ministry said.

Seoul demanded an explanation and called on its neighbour to give prior warning the next time such a situation occurs.

The incident threatened to cast a shadow over relations that had lately been improving after more than a year of tensions. The North last month freed five South Korean detainees, eased border curbs and sent envoys for talks with President Lee Myung-Bak.

In an unusually quick response, Pyongyang later Monday blamed a sudden surge in water levels for the discharge, according to the unification ministry.

In a message sent through an inter-Korean hotline, the communist state promised to give prior warning to the South if dam water is released in future, the ministry said in a statement.

However the North Korean message failed to impress South Korean officials.

"We deeply regret that the North Korean notice is not sufficient enough to convince us and it had no mention of the loss of human lives on our side," the unification ministry was quoted by South Korean agency Yonhap as saying.

Seoul's land ministry said 40 million cubic metres (52 million cubic yards) of water was released.

Some 2,600 soldiers and police were mobilised Monday to search for the missing. Police said three bodies had been recovered several miles downstream and a search was continuing for the other three.

Police are also investigating whether there was any negligence by Seoul officials in charge of issuing flood alerts.

On Sunday they found the body of a North Korean boy aged four or five which apparently drifted down river following the same incident.

The ruling Grand National Party demanded an apology.

"Whatever the reason was, it was an unpardonable criminal act," the party said.

"North Korea must make an immediate and official apology over the incident and present countermeasures to prevent it from recurring."

Problems began after the power-starved North began building dams on the Imjin river in 2000 to generate electricity.

Previous discharges from the dams have damaged fish farms and riverside areas in Yeoncheon County, 60 kilometres (37 miles) north of Seoul, but Sunday's was the first to claim any lives.

"We have yet to identify it as an attack by flooding," said defence ministry spokesman Won Tae-Jae.

Seoul officials have in the past expressed fears of such a deliberate attack.

In 1986 North Korea broke ground on a major dam near Hwacheon, about 200 kilometres northeast of Seoul.

South Korea began building its Peace Dam downstream in 1987 to block any discharge but the structure was not completed until 2005.

The two Koreas, who have remained technically at war since their 1950-53 conflict, have no formal accord on water discharges.

Seoul has repeatedly asked for pre-notification. But Pyongyang said it cannot give this because water from the dams is "naturally discharged when it reaches the maximum height," according to the unification ministry.