North Korea discharged water from a dam near the border earlier this week without notifying South Korea, officials said Wednesday.
The Ministry of Unification expressed regret over the discharge, saying the North ignored an inter-Korean agreement.
According to the Korea Water Resources Corporation (K-water), water flowing from the North's Hwanggang Dam on the Imjin River to the South's Gunnam Dam drastically increased from the average of 96 tons a second to 428 tons from 7 p.m. Monday to 4 a.m. Tuesday.
The Imjin River flows out to South Korea's west coast.
In response, Seoul opened its Gunnam Dam to control the water level. The dam, which began operations in 2010, was designed to deal with flash floods from Pyongyang.
K-water said the water swept away the nets and fishing gear of fishermen in Paju, Gyeonggi Province. It said it was unclear why the North discharged the water.
Some military observers raised speculation that it might have been the North's premeditated "water attack" aimed at checking the South's ability to react.
Unification Ministry spokesman Jeong Joon-hee said the North should stop discharging water without notice, saying the two Koreas agreed in October 2009 to notify each other of discharges.
"The North should never do it again as it is linked to the safety of the life and property of our people," Jeong said. "The North should issue prior notice before discharging water."
Regarding speculation over the North's water attack, Jeong said the ministry needed to check more details.