SEOUL, South Korea — Six South Koreans camping and fishing along a river flowing from North Korea were swept away Sunday when it suddenly doubled in height, apparently because a new dam in the North released a large amount of water without warning, officials said.

South Korean officials said they will ask for an explanation from Pyongyang, adding they had already conveyed concerns about the dam's effect on the South in recent years but received no response.

About 1,300 rescuers had failed to find those missing about 10 hours after the search began, fire official Park Ju-il said. He said the campers included a 12-year-old boy.

The Imjin River's water level jumped from 7.5 feet (2.3 meters) to 15.1 feet (4.6 meters) Sunday morning, Gyeonggi provincial official Choi Kwon-rak said. The Ministry of Land, Transport and Maritime Affairs said the North is believed to have released about 40 million tons of water starting at 2 a.m. (1700 GMT) on Sunday.

Choi said there was no other apparent cause since it had not rained in the area for several days.

South Korean officials have raised concerns the Hwanggang Dam could cause water shortages or flooding in the South, with some saying the North could use it as a weapon. Seoul's Unification Ministry, which handles relations with Pyongyang, said in a statement that it conveyed such concerns to North Korea during talks in 2006 and 2007, but the North hasn't responded.

The ministry plans to send a message to the North on Monday to request an explanation of what happened and to express regret at Seoul not be alerted to the suspected discharge of water, a ministry official said on condition of anonymity citing department policy.

Choi said the area is less than six miles (10 kilometers) from the Korean border – one of the world's most heavily fortified. The two Koreas are still technically at war because the 1950-53 Korean War ended with an armistice, not a peace treaty.

Construction of the dam, believed aimed at channeling water to a hydroelectric plant and for crop irrigation, was about 95 percent complete early last year, according to South Korean government estimates. The dam is believed to be able to hold up to 400 million tons of water.

Also Sunday, the corpse of what appeared to be a North Korean boy was found in the river's upper waters, about three miles (five kilometers) north of where the South Koreans went missing, local police officer Yu Sung-san said.

He said South Korean soldiers on routine patrol found the body at a site where civilians are banned. The boy's clothes had no labels, like many North Korean garments. HYUNG-JIN KIM