Almost 500,000 people have been ordered or advised to evacuate their homes in south-west Japan after torrential rain triggered widespread flooding.

Key points: At least 11 people missing, including a child

At least 11 people missing, including a child 774 millimetres of rain falls in parts of Fukuoka prefecture in nine hours

774 millimetres of rain falls in parts of Fukuoka prefecture in nine hours 7,500 rescuers sent to the area

Landslides were reported in several areas, and one man was dug out from the mud without signs of life, public broadcaster NHK said.

At least 11 people were missing or could not be reached, including a child, and many more were stranded and calling for help across areas of Fukuoka and Oita prefectures on the Japanese island of Kyushu.

Children and teachers at a school were among those cut off by the floodwaters.

Japan's weather bureau said the amount of rain in the region had broken all records and was continuing to fall.

Parts of Fukuoka prefecture were hit by 774 millimetres of rain in nine hours on Wednesday, more than two times the amount of rain that falls in a normal July, NHK said.

The massive landslides caused by the flooding left at least two houses swept away.

Record amounts of rain have fallen in parts of south-western Japan in recent days. ( AP: Sadayuki Goto, Kyodo News )

Some 7,500 rescuers, including police, firefighters and soldiers from Japan's Self Defence Forces, were mobilised to help with evacuations and search for the missing.

Officials said the military presence could be expanded up to 5,000 people and 50 helicopters if needed.

Residents flee to higher ground

Chief Cabinet Secretary Yoshihide Suga said: "There are many reports of people whose safety cannot be confirmed, things like 'a child was swept away by the river' and 'my house was swept away and I can't get in touch with my parents'."

"We will keep in close contact with the disaster-hit areas and work with all our energy to save lives and ascertain the extent of the damage," he told an early morning emergency early morning news conference.

Fukuoka and Oita prefectures, both largely rural areas, were the worst-hit by the rain, which was caused by a low pressure area on the Pacific Ocean that fed warm, moist air into Japan's seasonal rainy front.

Residents spent a worried night at evacuation centres set up at schools and government buildings on high ground.

The flooded area in Asakura, Fukuoka prefecture, south-western Japan. ( AP: Nozomu Endo, Kyodo News )

"I heard this tremendous rumbling noise and then the house exploded. A tree burst through the wall into the room," a man told NHK.

A schoolboy sitting with his family told NHK: "I haven't heard from some of my friends, and I'm really worried."

There were no immediate reports of major transportation problems, but television footage showed a railway line left broken and twisted and roads swept away by floodwaters.

All operations were stopped at a Daihatsu Motor plant in Oita because road conditions stopped staff and parts getting to the plant.

The same area was pounded by heavy rain earlier this week from Tropical Storm Nanmadol, which has since passed out to sea.