Twelve people died when a mudslide hit an inn and three homes in the mountainous Chuncheon area 100 kilometres (60 miles) east of Seoul early Wednesday, the National Emergency Management Agency (NEMA) said.
Five people were killed in a mudslide at Bangbaedong in southern Seoul Wednesday and one at Umyeon mountain in the city, which was hit by 380 millimetres (more than 15 inches) of rain since Tuesday.
Search and rescue efforts by more than 500 firefighters and police were still under way at Chuncheon.
Ten 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.
In Seoul, police declared a special emergency and more than a thousand officers were mobilised to help ease gridlock and block roadways deemed too dangerous.
Streets, subway stations, underpasses and residential districts were flooded throughout the capital. At Uijeongbu north of Seoul, Yonhap said three people were reported missing in flooded streams and rivers.
“We are overwhelmed by so many rescue requests and it’s not easy to keep track (of the damage),” a NEMA official told AFP.
The agency said floods or mudslides shut 36 major roads nationwide, including 23 in Seoul, and more than 700 homes were flooded in the capital. Dozens of others in the city could not leave home because of landslides or flooding.
News reports said more than 150 traffic lights were malfunctioning in Seoul, worsening the traffic gridlock, while mobile phone networks were cut off in some areas.