At least four people have died and over 1,300 others have been forced to evacuate as unrelenting downpours batter South Korea, prompting the government to raise its weather disaster alert to the highest level.

Authorities have warned the rain will continue, with widespread flooding already damaging homes, vehicles and businesses.

Among those confirmed dead are two men in their 80s, one believed to have drowned while attempting to drain water from his basement.

A third man died after a wall collapsed on his car shortly after he had called his wife saying his vehicle was "being swept away". The fourth victim suffered a cardiac arrest.

In Seosan, the worst-hit area on the country’s west coast, more than 400mm of rain fell in just 12 hours, described by the weather agency as a once-in-a-century event.

Social media was flooded with videos and photos showing submerged homes, floating furniture, and stranded vehicles.

“I couldn’t understand how something like this could happen,” said Kim Ha-min, a café owner in Gwangju, where 426mm of rainfall was recorded on Thursday. She described the stench from the sewers after floodwaters entered her shop: “I drained the water, but the smell is so bad that I can’t open for business.”

Another online user from a heavily flooded area posted: “Everything is covered by water except the roof of my house.”

By 4pm local time on Thursday, rescue teams had evacuated over 1,300 people across affected regions, with several injuries also reported. Two people were treated for hypothermia and two others sustained leg injuries.

Authorities have urged residents to avoid riverbanks, hillsides and underground spaces due to the ongoing threat of flash floods and landslides.

The meteorological department explained that the extreme weather was caused by dry air from the northwest mixing with warm, humid air from the south, forming unusually large rain clouds. Forecasters say while the rain is expected to ease, a heatwave could follow in the coming week.