Rescue workers take part in an operation near an underpass submerged by a flooded river in Cheongju. Photo: Reuters

Five bodies of people trapped under a tunnel submerged by heavy rains in central South Korea have been recovered, firefighting authorities say, taking the death toll from the days of torrential rains that pounded the country to 31.

The five people were found from a bus that has been submerged at a flooded underpass in Cheongju on Saturday after rescue workers began underwater search operations early on Sunday, officials said.

The Ministry of Interior and Safety earlier said 26 people have died and 10 were missing as of 6am on Sunday (local time) as the heavy rains caused landslides and floods across the country, with evacuation covering 7540 people.

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.

"We are focusing on the search operation as there's likely more people there," Seo Jeong-il, head of the west Cheongju fire station, told reporters.

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.

More heavy rain was expected on the Korean peninsula on Sunday.