At least 13 people were killed on Sunday when a South Korean fishing boat collided with a refueling vessel, the coast guard said.
The collision caused the boat, to capsize, it added.
Seven people were rescued, while two remain unaccounted for, including the captain, stated an official from the Korea Coast Guard.
He explained that 22 people were aboard the 9.8-ton fishing boat that capsized after colliding with the 336-ton refueling vessel in waters off the port city of Incheon.
The official spoke on condition of anonymity, citing office rules. The refueling vessel did not suffer damage.
Those rescued from the water have been sent to nearby hospitals, the coast guard added.
President Moon Jae-in ordered authorities to deploy as many helicopters and other aircraft as possible to search for the missing, according to his office.
The coast guard official said 19 coast guard and naval vessels and five aircraft including helicopters were dispatched to the site. Authorities were questioning the crew of the refueling vessel to determine the cause of the collision.
South Korea has seen its share of significant maritime accidents in recent years, including the 2014 sinking of a ferry that killed more than 300 people, mostly schoolchildren. More than 50 fishermen died or went missing months later after their vessel sank in the Bering Sea.