A typhoon menacing already sodden southwestern Japan could dump weeks' worth of rain on the area, forecasters warned Wednesday, as communities struggle to recover from huge floods.

Typhoon Khanun was sitting just west of the main southern island of Kyushu, packing winds of up to 126 kilometres (78 miles) per hour, the Japan Meteorological Agency said.

Although a direct hit looked unlikely, up to 15 centimetres (about six inches) of rain were forecast in 24 hours for Oita and Kumamoto prefectures, where floods and landslides have left 32 dead or missing over the last week.

"In some places recent heavy rainfall has left the ground very loose, so we are warning local residents to be vigilant against mudslides," the weather agency said.

More than 90 centimetres of water has fallen in the area around Mount Aso, an active volcano, so far this month, the agency said, far outstripping what would normally be expected.

Around 15 people in the small city of Ukiha were cut off when roads were damaged and bridges washed away after rivers burst their banks.

"I have never experienced such floods in my life," 57-year-old farmer Shigeru Kawauchi told AFP.

"It is the first time I have seen the river flowing so violently that it was sweeping houses away," he said.

Khanun is expected to near South Korea's Jeju island by Wednesday night.

Meanwhile the main Japanese island of Honshu was wilting under soaring temperatures, with authorities issuing hot weather alerts in central parts of the country.

A 55-year-old man died in Toyama prefecture apparently from heat stroke on Tuesday, while 688 people were taken to hospital due to heat exhaustion, public broadcaster NHK said.

One person died and nearly 700 others were taken to hospital suffering from the effects of heat on Monday.

Japan is asking people to limit the use of electricity-sapping air conditioners as the country battles a possible power shortfall, with the vast bulk of nuclear generators offline in the aftermath of the Fukushima disaster.