At least 13 people were killed after a fishing vessel collided with a 336-ton oil tanker and He overturned. South Korean coast.
Two more were missing, said the South Korean coast guard, while a search and rescue operation continued.
The rented fishing boat, the Seonchang-1, carried 20 pbadengers and two crew members during a fishing tour at the time of the accident.
Images from the scene showed that several divers were looking for the overturned boat.
Navy helicopters and dozens of ships participated in the search southwest of Incheon, near Yeongheung Island.
Seven people were taken to the hospital for treatment. The captain of the 10-tonne fishing vessel was among those missing, according to an AFP report.
No injuries were reported on the 336-ton tanker.
The South Korean news agency Yonhap said the collision occurred nine minutes after the ship left the coast, possibly when the two ships crossed under a bridge.
"There is no specific problem related to weather conditions, navigation reports or other preparations (pre-departure)," a coastal guard officer told reporters. "We are investigating how the accident happened."
Cold water temperatures may also have contributed to the victims, the official said.
The accident is believed to be the worst in South Korea, since 15 people died on a fishing trip near Jeju in 2015.
The previous year, a pbadenger ferry capsized and killed more than 300 people , most of them school children on a field trip.
The ship, the Sewol, was lifted from the bottom of the sea earlier this year.