SEOUL - South Korea demanded an apology from North Korea on Tuesday for its sudden release of water into a river flowing across their border that swept away six people, casting a pall over recently warming ties between the rivals.

South Korean emergency officials have recovered the bodies of three of the six, who went missing in a pre-dawn flash flood on Sunday caused by the sudden surge of water on the Imjin River, where the North has used massive dams to control water flow.

The six, including a child, were camping on the Imjin River's banks.

North Korea said late on Monday that it had to release water due to rising water levels on its side of the river and said it would notify the South of such actions in the future.

This is not an acceptable message and the government finds it very regrettable that there is no mention whatsoever of the grave human casualty on our side, Unification Ministry spokesman Chun Hae-sung said in an official government response.

The government demands a full explanation and an apology by a responsible member of the North's government about the unannounced release of water that resulted in human casualty.

North Korea has made conciliatory gestures in recent weeks to the South after months of fiery rhetoric against the government of President Lee Myung-bak who came to office last year and stopped years of lucrative aid to the communist neighbor.

(Reporting by Jack Kim; Editing by Jonathan Hopfner)