AT least 36 people were killed and dozens missing today after the heaviest rains in decades pounded South Korea, triggering landslides and inundating hundreds of homes.

As main roads turned to rivers of churning, muddy water, the National Emergency Management Agency (NEMA) said the death toll at 11pm local time had climbed to 36 and was expected to rise, with dozens of people still missing, Yonhap news agency reported.

Thirteen people died when a mudslide hit an inn and three homes in the mountainous Chuncheon area 100km east of Seoul overnight, NEMA said.

At least 17 people were killed the same day in southern Seoul by landslides from Umyeon mountain in the city, which was hit by 433.5mm of rain in 40 hours between Wednesday and today.

Hundreds of people were forced to evacuate their homes.

A river in Gonjiam, about 30km southeast of Seoul, overflowed, killing three residents, the emergency agency said.

A landslide just north of the capital killed three factory workers, Yonhap said, citing media reports.

Search and rescue efforts by more than 500 firefighters and police were still underway at Chuncheon, where 10 of the victims were students from Inha University in the western city of Incheon, who were in the area for volunteer work.

"I was sleeping on the second floor of the pension (inn) when I heard the thunderous sound of a landslide. The stairs collapsed and I was buried under mud," one student rescued by firefighters told Yonhap news agency.

NEMA said 20 others were injured, four of them seriously.

media_camera Residents on a makeshift raft evacuate from a flooded area in Gwangju, South Korea. Picture: AP

In Seoul, police declared an emergency, and more than 4000 officers were mobilized to help ease traffic gridlock, block roads deemed too dangerous to use, and aid rescue efforts.

The military also dispatched 1500 soldiers to help with rescue work in the capital, Yonhap reported.

Streets, subway stations, underpasses and residential districts were flooded throughout Seoul.

"We are overwhelmed by so many rescue requests and it's not easy to keep track (of the damage)," a NEMA official said.

The agency said floods or mudslides shut 37 major roads nationwide, including 23 in Seoul, and more than 700 homes were flooded in the capital. Others in the city were meanwhile prevented from leaving home by landslides or flooding.

News reports said more than 150 traffic lights were malfunctioning in Seoul, worsening the jams, while mobile phone networks were cut off in some areas.

TV images showed main streets turned into a sea of muddy water dotted with the roofs of submerged cars, with some drivers abandoning vehicles to run to safety.

Apartments and houses struck by landslides were destroyed or buried under a mass of mud, with scores of destroyed cars tossed on top of each other and tangled with uprooted trees.

The Korea Meteorological Administration (KMA) forecast more downpours until Friday, with expected rainfall of up to 10 inches (250 millimeters) until later today in some areas in Seoul and nearby regions.

Rare downpours tallying more than 110mm of rain per hour were recorded in southern Seoul, a KMA spokesman said.

Originally published as Torrential rains kill 36 in South Korea