Rescue workers in South Korea are looking for six people believed to have been swept away from a campsite in a mysterious flash flood, BBC reported.
Officials say they are investigating whether the surge came from a new dam in North Korea, further up stream.
They say there has been no rainfall recently in the area at Yeoncheon, 60km (37 miles) north-east of Seoul.
More than 1,000 rescue workers are scouring a stretch of the Imjin river, close to the border with the North.
Although the cause of the flood has not been confirmed, the BBC's John Sudworth says concerns have been raised in the past that North Korean dams could cause both droughts and floods in the South, depending on the time of year.
Police said the six South Koreans disappeared early on Sunday from the campsite.
Provincial official Choi Kwon-rak said the water level suddenly jumped 2.3m (7.5ft).
A local police official told the AFP news agency that investigators suspected the Hwanggang Dam in North Korea had released a massive volume of water.
The dam has a capacity of 400 million tons of water, according to South Korean government estimates.
Seoul's Unification Ministry, which handles relations with Pyongyang, said it was looking into the incident.