South Korea’s Interior and Safety Ministry said three of those who died had been trapped in a flooded semi-basement. Nine others were injured and six people remain missing, the ministry said.
Since midnight Monday local time, parts of Seoul saw a total of 422 millimeters (16 inches) of rain, prompting authorities to raise the highest Level 3 emergency alert. The city recorded 141.5 millimeters (5.57 inches) of rain per hour — the highest rate since authorities began keeping records.
Photos from across the city show severe flooding, with people wading across roads up to their thighs in water.
Though floodwaters had largely receded by Tuesday morning, cars and buses were left strewn across roads and sidewalks, blocking morning traffic.
In some parts of Seoul, drains backed up and sent water pouring back into streets and subway stations, according to the Seoul Metro. A number of subway stations were closed due to flooding, with lines temporarily suspended Monday night. As of Tuesday morning, authorities were still working to reopen the stations.
Several regions south of the Han River were worst affected, including the wealthy, modern Gangnam district where some buildings and stores were flooded and lost power.
Around 800 residents were evacuated to schools and gyms or voluntarily sought shelter in local community centers as flooding affected more than 741 houses and shops, according to authorities.
South Korean President Yoon Suk Yeol sent his condolences to the victims on Tuesday, saying he would conduct an on-site inspection and work to prevent additional damage.
He also pointed out the need to review the country’s disaster management system, since extreme and unpredictable weather is expected to become increasingly common due to the climate crisis.
Heavy rain is expected to continue on Tuesday, with up to 100 millimeters (3.9 inches) of rain per hour in some regions, according to the country’s Meteorology Administration.
Parts of Japan also saw downpours on Monday night, with some regions of Hokkaido reporting flooding — but no injuries as of Tuesday. Authorities have warned of the risk of flash floods and landslides.
Additional reporting by CNN’s Jake Kwon and Reuters.