Originally Published: 09 AUG 22 00:22 ET

Updated: 09 AUG 22 11:37 ET

By Gawon Bae and Jessie Yeung, CNN

(CNN) -- At least eight people were killed in Seoul on Monday after record downpours flooded homes, roads and subway stations in the South Korean capital, cutting power and forcing hundreds to evacuate, according to authorities.

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 at least seven people remain missing, the ministry said.

Since midnight Monday local time, parts of Seoul saw a total of 422 millimeters (16.6 inches) of rain, prompting authorities to raise the highest Level 3 emergency alert. The city recorded 141.5 millimeters (5.6 inches) of rain per hour -- the highest rate since authorities began keeping records.

Additional rounds of heavy rain are forecast to move through Seoul until Thursday, which could bring additional flash flooding to the region.

Heavy rain warnings are in effect in the metropolitan area and Gangwon-do, where rainfall of 50 to 100 millimeters (2-4 inches) per hour is possible, according to the Korean Meteorological Administration.

Photos from across the city during Monday's flash floods show 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.

About 800 residents were evacuated to schools and gyms or voluntarily sought shelter in local community centers as flooding affected more than 700 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 weather is expected to become increasingly common due to the climate crisis.

Several rounds of heavy rain will hit Seoul Tuesday night and continue through Thursday morning before ending Thursday afternoon. An additional 300 millimeters (11.8 inches) will be possible over the already flooded areas before finally ending. The rain may add to further flooding, and mudslides are possible

Seoul typically averages 348 millimeters (13.7 inches) of rain in August -- the wettest month of the year there. Several locations recorded this much rainfall in just one day.

Many countries across East Asia are now experiencing more intense daily rainfall, with summer monsoons expected to grow stronger and more unpredictable as the Earth warms, according to the Intergovernmental Panel on Climate Change.

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.