The 9.77-ton South Korean vessel was carrying two crew members and 20 passengers when it hit the 336-ton tanker in the waters near Yeongheung Island at around 6:12 a.m. and capsized.
The number of dead, injured and missing are as of 1 p.m., a Coast Guard official said.
President Moon Jae-in received a briefing on the accident less than an hour after the incident and ordered all-out search and rescue operations, his office Cheong Wa Dae announced.
Authorities have mobilized four helicopters and 14 boats for the operation.
The Ministry of Oceans and Fisheries launched a task force to handle the accident.
"Cardiopulmonary resuscitation is under way for the rescued," a ministry official said. "We are trying to confirm their exact health conditions."
An informed source said there have been no reports of violations in connection with the boat operations, including the number of passengers on board. Source from the Yonhap.