The Imjin River's level suddenly doubled Sunday, sweeping away six South Koreans camping and fishing near the border dividing the two Koreas. South Korean officials quickly suspected the estimated 40 million tons of water came from a new North Korean dam - one that some in the South had warned the North could use as a weapon.
The North said in its message Monday that it "urgently" had to release the water because river levels were too high. It also said it would warn its neighbor of similar releases in the future, Seoul's Unification Ministry said.
Pyongyang's quick response suggests it does not want to strain recently improving relations with the South.
The river's water level jumped from 7.5 feet (2.3 meters) to 15.1 feet (4.6 meters) Sunday morning, local Gyeonggi Province official Choi Kwon-rak said.
Rescuers found three bodies early Monday and were searching for three others, local fire official Hwang Tae-geun said. Some 2,500 rescuers and army soldiers were mobilized Monday, he said. The area is less than 6 miles (10 kilometers) from the Korean border.
There were no immediate signs the water release was meant as an attack, Defense Ministry spokesman Won Tae-jae told reporters.
North Korea and South Korea technically remain in a state of war, divided by a heavily militarized border, because their three-year conflict in the 1950s ended in a truce, not a peace treaty.
Relations blossomed under the "Sunshine Policy" espoused by two liberal South Korean administrations but went cold after the election of conservative President Lee Myung-bak 18 months ago.
Pyongyang recently reached out to Seoul again, agreeing to restart long-suspended joint economic projects and resume the reunions of families divided during the Korean War.
Despite the overtures, "this is not something that we can overlook," the JoongAng Ilbo newspaper said in an editorial Monday. It warned that South Korea would suffer "unimaginable damage" if the North uses the dam for military purposes.
South Korean officials have raised concerns that the Hwanggang Dam could cause water shortages or flooding in the South, with some concerned the dam could be used as a weapon. Unification Ministry officials said South Korea conveyed such concerns to North Korea during talks in 2006 and 2007, but the North didn't respond.
As part of anti-flooding efforts, South Korea has been building two dams in the area, scheduled to be completed by 2011 and 2012, respectively. On Monday, the Ministry of Land, Transport and Maritime Affairs said in a statement it will advance the construction of the first dam by one year in the wake of Sunday's flooding.
The ministry said the North Korean dam is located about 26 miles (42 kilometers) north of the border, and intended to channel water to a hydroelectric plant and for crop irrigation. It is estimated to have a capacity of 400 million tons of water.
Associated Press Writer Jae-soon Chang contributed to this report.