SEOUL, South Korea  North Korea unleashed walls of water from one or more of its dams on Sunday, sending a flash flood roaring through the heavily armed border with South Korea and sweeping away six South Koreans, officials in Seoul said.

The six South Koreans, who are still missing, include an 8-year-old boy. They were camping or fishing on the Imjin River, which flows from the North, enters South Korea north of Seoul, and meanders close to the border before emptying into the Yellow Sea.

“The government will send a telegram to the North on Monday to protest and demand a satisfying explanation,” the Unification Ministry of South Korea said in a statement.

Officials said the Imjin River’s water level on the border doubled, to 15.1 feet, after North Korea began releasing water without warning at 2 a.m. on Sunday, either from its newly built Hwanggang Dam 17 miles north of the border or from a smaller dam downstream.