INCHEON/JEJU -- A Coast Guard helicopter crashed in waters off South Korea's southernmost island of Mara on Friday, leaving two crewmen dead and another missing, officials said.

Four people were aboard the Sikorsky S-92 helicopter when it went down in waters 370 kilometers southwest of Mara Island at 1:32 a.m., officials said.

Of them, a co-pilot, identified by his surname Chung, 51, and a vessel radar operator, surnamed Hwang, 28, were killed, and a mechanic, surnamed Cha, 42, went missing. Its pilot, surnamed Choi, 47, was rescued by a nearby Coast Guard patrol boat and is in a non-life threatening condition, officials said.

The crash happened shortly after the helicopter took off to return to Busan after dropping off six Coast Guard rescuers on the patrol boat that was on its way to help with rescue operations for crew members missing from a tug boat in waters near Taiwan.

"Following the helicopter's crash, the patrol boat's crew rescued three people and two of them died, while the remaining one remains missing," a Coast Guard official said.

Currently, 17 ships, including seven Coast Guard patrol boats, a Navy warship and private fishing boats, are searching the area to rescue the missing crewman. (Yonhap)