North Korea intentionally caused a flash flood that killed six South Koreans south of the border, a Seoul minister said Wednesday.

Asked whether Sunday's discharge of dam water into a cross-border river was intentional or a mistake, Unification Minister Hyun In-Taek told parliament: "I think the North did it intentionally."

Hyun's comments, reported by Yonhap news agency, were confirmed by his ministry. He is the first South Korean official publicly to claim that the release of the water was deliberate.

North Korea on Monday blamed a sudden surge in the dam's water level for the "emergency" release and promised to give prior warning of future discharges.

But Seoul has demanded an apology and full explanation from Pyongyang for the sudden surge, which carried away campers south of the heavily fortified border.

Officials said there had been no heavy rain in the North in recent days to explain such a surge, and also noted its communist neighbour failed to express regret for the victims.

The tragedy threatens to damage cross-border relations which had lately been improving.

Last month the North made peace overtures both to its southern neighbour and the United States. It freed five South Korean detainees, eased border curbs and sent envoys for talks with President Lee Myung-Bak.

Some Seoul newspapers have raised suspicions the surge was intentional and possibly aimed at pressuring the South to resume cross-border aid and economic projects.

Police recovered three more bodies from the Imjin river on Wednesday, in addition to three recovered earlier. One of those found Wednesday was a boy aged eight.

Seoul said an estimated 40 million cubic metres (52 million cubic yards) was released from the North's Hwanggang Dam.

Defence Ministry officials said surveillance over North Korean dams upstream would be strengthened to prevent any recurrence and cope with any "water attacks."

Aircraft and a pilotless drone would be used to keep watch.

The power-starved North started building dams on the Imjin river in 2000 to generate electricity.

Seven previous unannounced discharges have damaged fish farms and riverside areas south of the border but Sunday's was the first to claim lives.

The Hwanggang (Hwang River) dam was completed in February this year, the unification ministry told a parliamentary committee. It can hold up to 400 million cubic metres of water.