Another boat nearby raised the alarm after reporting the 129-ton ship was going under at around 4.30am, 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 hospitalised, 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 mobilising all rescue equipment and resources,” Chung told a televised briefing.

One of the rescued sailors told the 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.

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 defence ministry to supply military flares and aircraft to support night operations.