South Korean rescuers have pulled nine bodies from a flooded tunnel where around 15 vehicles were trapped in muddy water, as days of heavy rain triggered flash floods and landslides and destroyed homes across the country, officials said.

A total of 37 people have died following continued rescue efforts on Sunday and thousands have been evacuated since July 9, when heavy rain started pounding South Korea's central regions.

Nearly 900 rescuers including divers were searching the tunnel in the central city of Cheongju, where the vehicles, including a bus, were swamped by a flash flood Saturday evening, Seo Jeong-il, chief of the city’s fire department, said in a briefing.

Fire officials estimated that the tunnel filled with water in as little as two or three minutes.

Photos and video from the scene showed rescue workers establishing a perimeter and pumping brown water out of the tunnel as divers used rubber boats to move in and out of the area.

Yang Chan-mo, an official from the North Chungcheong provincial fire department, said it could take several hours to pump out all the water from the tunnel, which was still filled with 4 to 5 metres of water dense with mud and other debris. Workers were proceeding slowly to prevent any victims or survivors from being swept out, Yang said.

Nine survivors were rescued from the tunnel and around 10 others were believed to be missing based on reports by families or others, but the exact number of passengers trapped in vehicles wasn’t immediately clear, Seo said.

"I have no hope but I can't leave," a parent of one of those missing in the tunnel told local news agency Yonhap.

"My heart wrenches thinking how painful it must have been for my son in the cold water."

The casualties from the flooded tunnel were not included in the ministry's data as it was not immediately clear how many people and cars were trapped underwater.

President Yoon Suk-yeol, now on an overseas trip, ordered Prime Minister Han Duck-soo to mobilise all available resources to minimise the casualties, his office said, as more heavy rain was expected on the Korean peninsula on Sunday.

Record breaking floods

South Korea is regularly hit by flooding during the summer monsoon period, but the country is typically well-prepared, and the death toll is usually relatively low.

The country endured record-breaking rains and flooding last year, which left more than 11 people dead.

They included three people who died trapped in a Seoul basement apartment of the kind that became internationally known because of the Oscar-winning Korean film "Parasite".

The government said at the time that the 2022 flooding was the heaviest rainfall since Seoul weather records began 115 years ago, blaming the climate crisis for the extreme weather.