ASAKURA, At least one person is dead and 18 others are missing in huge floods that are surging through southern Japan, with authorities warning hundreds of thousands of people to flee.

Unprecedented torrential rain has caused rivers to burst their banks, sweeping away roads and houses, and destroying schools.

Thousands of soldiers and other rescuers were scrambling Thursday to reach those cut off by torrents of swirling water or threatened by landslides, as forecasters warned of worse to come.

"We are in an extremely serious situation," Deputy Prime Minister Taro Aso said, warning of the danger of collapsing hillsides and adding "many people are still missing".

More than 50 centimetres (20 inches) of rain deluged parts of Kyushu, the southernmost of Japan's four main islands, over 12 hours on Wednesday, the meteorological agency said.

Downpours will likely continue through Friday, the agency said as the region grapples with the aftermath of a typhoon that raked the country this week.

Japan is deploying 7,500 police, rescue personnel and troops in affected areas of Kyushu, Chief Cabinet Secretary Yoshihide Suga said.

"There is a lot of information about people who are unaccounted for," he told a press conference, declining to confirm the number of people missing.

Unable to escape

Those included a child reportedly carried off by a river in spate and a couple who had not been seen since their house was swept away.

A man was found dead in the city of Asakura in Fukuoka prefecture, public broadcaster NHK and Jiji Press reported, citing police sources.

Local officials said in total they knew of 18 people who were missing in Fukuoka and Oita prefectures.

An official in Hita, in Oita prefecture, said a landslide left three people injured, one critically.

Television footage showed rolling waves from swollen rivers hitting residential areas, tearing up roads and inundating farmland.

Asakura was among the hardest hit with footage showing floodwaters surging through the streets.

"My daughter and son are still at home, unable to escape outside," an exhausted middle-aged woman at a shelter in Asakura told NHK.

A railroad bridge was destroyed by the raging Kagetsu river, disrupting train services, a railway spokesman said.

Several other train lines were also forced to delay or stop operations due to heavy rains, while local officials called off classes at primary and middle schools.

-AFP