INCHEON -- A fishing boat capsized in waters near a northwestern border island Saturday, leaving three crew members missing, the Coast Guard said.

The 12-ton vessel was overturned in waters 43 kilometers from the border island of Soyeonpyeong at 6:07 p.m., the authorities said. The cause of the capsizing was not immediately known.

Three crew members, including the 63-year-old captain, went missing, while two others were rescued. One of the rescued remains unconscious, officials said.

Prime Minister Chung Sye-kyun directed the Coast Guard and the Navy to mobilize all available resources to find those unaccounted for.

Interior and Safety Minister Chin Young also called for "all-out" efforts to rescue the missing, while instructing officials to provide necessary support to their families, such as providing information on the status of the search operations.

The Coast Guard and the Navy mobilized 17 ships and five aircraft to search for the missing. Twenty-three fishing boats also joined the search operations.

"We have formed a search headquarters, which is using illuminating shells to find the missing," a Coast Guard official said.

"As soon as the search operation is over, we will investigate the cause of the accident," the official added.

The scene of the accident is 23 km southwest of the Northern Limit Line, a de facto inter-Korean sea border. (Yonhap)