This photo provided by the Coast Guard's Jeju office shows a capsized boat in waters off Mara Island, Oct. 18. Yonhap

A fishing boat capsized in waters off Mara Island, just south of the southern island of Jeju, on Tuesday, Coast Guard officials said, adding that a search is under way to rescue four crewmen believed to have been on board.

The 29-ton fishing boat capsized in waters 7 kilometers southwest of Mara Island at 3:13 a.m., according to a report filed with the Coast Guard.

Coast Guard rescuers arrived at the scene around 5 a.m. and found the boat upturned. Mobilizing a patrol ship, the Coast Guard began a search to rescue the four crewmen who were said to have been aboard, the officials said. (Yonhap)