North Korea told South Korea that it may release water from its border dams starting Thursday morning, the second such notice in five days.
A South Korean Unification Ministry official, asking not to be named, said it could not be immediately confirmed whether the water level near the border had begun to rise.
"Discharges of water from dams north of Imjin River are expected to begin at 6 a.m.," the North Korean notice was quoted as saying in a ministry statement.
The notice came through a military communications line on the west coast at 6:40 a.m. on Thursday, South Korean Unification Ministry spokesman Chun Hae-sung said.
Last September, six South Korean campers died when North Korea suddenly discharged water from one of its border dams, triggering a flash flood. The North expressed regret and promised to notify the South of future discharges.
On Monday, North Korea discharged 1,000 tons of water per second from one of its border dams, after having informed South Korea a day earlier that it would do so.
South Korea warned against reading the North Korean move as a gesture aimed at thawing relations, as tensions remain high on the peninsula after a South Korea-led multinational probe found North Korea responsible for the deadly March sinking of a warship.
Torrential rains have pounded the Korean Peninsula for the past few days, raising the water levels of major rivers. The peninsula is currently in the grip of the summer rainy season.