Japanese rescuers are continuing their search for victims of freak rains that have triggered floods and landslides in southwest Japan and left at least 18 people dead and hundreds of others displaced, public broadcaster NHK reported on Sunday.

According to Reuters news agency, about 1,900 policemen and soldiers using heavy machinery have braved the rain and grappled with debris of driftwood and mud that have cut off roads and clogged flooded homes.

Authorities warned of more heavy rains later in the evening and potential landslides. The downpour has been caused by a low pressure over the Pacific that has sent warm, moist air into Japan's seasonal rainy front.

In Fukuoka and neighbouring Oita, the hardest hit areas, 18 people have been killed and 14 have been injured, while roughly 570 have been left isolated and more than 20 are still unaccounted for, NHK said.

Reuters said that the city of Asakura has been hit by more than 600mm, or 24 inches, of rain since the downpour started on Wednesday, the Japanese meteorological agency said, warning of yielding ground.

The meteorological agency said the rainy front was forecast to bring 120 mm of rain in the northern Kyushu region by Monday noon, and 100mm of rain in the neighbouring Chugoku region, western Japan.

Some places have seen more rain in a matter of hours than they usually get in the whole month of July.