One Coast Guard officer died and three others went missing after a helicopter on a rescue mission crashed near an island off South Korea’s southwestern coast, officials said Saturday.

The Coast Guard helicopter crashed in waters off Gageodo Island at around 8:27 p.m. Friday, while trying to land near a seawall to carry out a mission to transport an emergency patient to an inland hospital.    

Of the four aboard the helicopter -- two pilots, one helicopter mechanic and a relief worker -- the mechanic was rescued late Friday and rushed to a nearby hospital, but was pronounced dead, according to officials.

"The Coast Guard officials and the Navy have launched operations to search for those unaccounted for," a government official said.

"Strong waves and heavy sea fog have effectively hampered overnight search operations. More rescue vessels, maritime patrol aircraft, and naval ships will be mobilized," he added.

A thick sea fog reported in the area on Friday may have caused the accident, with an investigation into what exactly happened currently underway, according to the authorities. (Yonhap)