Rescuers were racing against time Friday in the search for people buried under mudslides as heavy rains that have already claimed 20 lives continued to pound southwestern Japan.

Police, firefighters and troops were digging through mud and rubble with shovels as more torrential rain fell, on top of "unprecedented" downpours that swamped whole neighbourhoods on Thursday.

Television footage showed torrents of muddy water carrying uprooted trees and other debris after Thursday's violent deluge, while rivers burst their banks and flooded towns and villages in the main southern island of Kyushu.

Reports showed residential streets in the city of Kumamoto covered in mud, while battered cars that had been swept away by flood water were left dumped on hillsides in scenes reminiscent of the March 2011 tsunami in the northeast.

In the city of Aso in Kumamoto prefecture, central Kyushu island, landslides buried at least 17 households, killing 19 people with six more still missing, local officials said.

"We will keep searching for the missing throughout the night, while urging our citizens to stand guard as heavy rain continues sporadically," said Kumamoto prefectural official Yushin Maekawa.

In Oita, a man in his 70s died after being swept into a raging river, while another man remained missing.

Public broadcaster NHK showed rescuers continuing their search, using heavy machinery to remove uprooted trees, boulders and debris as night fell.

The weather eased somewhat Friday morning bringing temporary relief, but further downpours were recorded in Kyushu later in the day.

The city of Hita, Oita prefecture, issued evacuation orders to more than 14,800 people amid increasing fears the Kagetsugawa river, which runs through the city, would burst its banks, local officials said.

Nearly 25,000 people in other parts of the prefecture were advised to seek safety, the officials said.

In neighbouring Kumamoto prefecture, more than 250 households were told to evacuate, while everyone living on the floodplain of the Shirakawa river in Kumamoto city was advised to get to safety.

Friday's downpours were set to add to the misery for an area where 50,000 people were ordered to leave their homes on Thursday after it was lashed by the heaviest rainfall on record.

Landslides and fallen trees have cut roads and water supplies in several places, with at least one small mountain community completely cut off, the local government said.

A Japanese military helicopter dropped supplies including food and water onto the isolated district, it said.

The weather agency urged residents of Kyushu to be vigilant against mudslides and floods after rainfall of 10.1 centimetres (about 4 inches) per hour was recorded in the southern region of Kagoshima early Friday.

The weather agency forecast further about 20 centimetres of rainfall in the northern Kyushu region during the 24 hours to 6:00 am Saturday (2100 GMT Friday).