SEOUL, Jan. 28 (Yonhap) – A small fishing boat carrying one South Korean and two Indonesian fishermen has sunk off the southern island of Jeju, with only one Indonesian rescued and the two others missing, the Coast Guard said Sunday.

The rescued Indonesian fisherman is under medical treatment at the Jeju National University Hospital for symptoms of hypothermia.

Four Coast Guard aircraft conducted an overnight search operation, using 122 illumination bombs to light up the waters, about 18.5 kilometers off Jeju's southeastern district of Pyoseon, to try to find the South Korean captain and the other Indonesian fisherman.

A search operation is currently under way with the help of 10 patrol ships as well as other Navy and private fishing boats.

Still, the two fishermen remain unaccounted for as of Sunday morning.

The Jeju Port Vessel Traffic Service Center reported to the Coast Guard around 9:52 p.m. on Saturday that the 4.11-ton fishing boat was sinking

When Coast Guard rescuers arrived at the scene, the vessel was nowhere to be seen.

The Coast Guard plan to look into the exact cause of the sinking as soon as the rescued fisherman's health condition stabilizes.