A South Korean fishing boat capsized off the country's western coast on Sunday after it collided with a refueling vessel, killing eight people, South Korea's coast guard said.

The boat was carrying two sailors and 20 passengers when the collision occurred in waters near Incheon west of Seoul, according to the coast guard.

Two people remain missing and a search and rescue operation involving five helicopters and 19 vessels is underway. The reason for the collision has yet to be confirmed, the coast guard added.

South Korea's President Moon Jae-in visited a national emergency management centre and ordered all measures be taken to find those missing, the Presidential Blue House said.