There was a loud impact and noise before the ferry off a coast of South Korea began sinking, a passenger who had been rescued from the vessel told YTN news television.
One person has been found dead in the ferry wreckage, according to a coast guard official.
The president of South Korea, Park Geun-hye, earlier ordered all available vessels and special operations divers to the rescue operation.
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Rain will spread eastwards across southern areas tonight. Clear and frosty in the north.