According to the Associated Press, the plane crashed after reporting a mechanical problem and losing contact with air traffic workers, airline official Kim Dong-won said.
Five coast guard patrol boats and four helicopters were searching the area for signs of the pilot and co-pilot, Jeju coast guard spokesman Choi Kyu-mo said.
Asian Airlines said in a statement which is quoted by Associated Press, Asiana Airlines cargo plane had taken off from South Korea's Incheon International Airport, west of Seoul, and was bound for Pudong in China. It was carrying computers, semiconductors, paint and resin solution among others, it said.
