Two people have died and 12 are missing after a fishing boat sank on Friday in waters off South Korea's southern resort island of Jeju, officials said.

Another boat nearby raised the alarm after reporting the 129-tonne ship was going under at around 4:30 a.m. (1930 GMT on Thursday), prompting authorities to launch a rescue operation, said Chung Moo-won, a Jeju coast guard official.

Of the 27 crew aboard, 15 were rescued, including two South Korean nationals who later died after being hospitalized, Chung said. Twelve people were still missing, two of them from Indonesia.

A total of 43 vessels including three navy ships, 13 aircraft and several deep-sea divers were dispatched to assist in the rescue efforts, he said.

"We offer our condolences to the bereaved families and will make utmost efforts to find the missing by mobilizing all rescue equipment and resources," Chung said in a televised briefing.

One of the rescued sailors told Yonhap News Agency that the boat capsized when they were hauling up fishing nets to transfer the catch to another vessel. 

Chung said authorities would look into the exact cause of the accident by interviewing survivors.

South Korean President Yoon Suk-yeol ordered all available resources to be deployed for the operation while ensuring the rescue workers' safety, his office said in a statement.

Prime Minister Han Duck-soo also called for the Defense Ministry to supply military flares and aircraft to support night operations.