A tugboat was believed to have sunk off an island near South Korea's southernmost island of Jeju for unknown reasons on Thursday, leaving its two crew members dead, the Coast Guard said.

The coast guard in the city of Jeju received a transmission from an Emergency Position Indicating Radio Beacon, or a distress message suggesting a ship's sinking, from the 29-ton K tugboat sailing in waters 2.4 kilometers west of Biyang Island near the northwestern coast of the southern resort island around 8:51 a.m.

Following the receipt of the signal, the tugboat's captain, 71, was rescued by a ship sailing nearby and the other crew member, whose age is unknown, was picked up by a patrol boat before the two were pronounced dead at a hospital.

The coast guard sent four patrol boats and one helicopter to the site to search for the tugboat, which left a port in Hallim, Jeju, around 1:10 a.m. The accident occurred during strong winds and high waves. (Yonhap)