At least 18 people have been killed in Japan when landslides caused by torrential rain hit the outskirts of the city of Hiroshima.

At least 13 people are missing after a month's worth of rain fell overnight.

Rescue workers are searching through mud and piles of stones in residential areas about five kilometres from the centre of the city.

Among those dug out of the debris was a two-year-old boy who was not breathing but it was not clear if he had been killed, media reported.

With land in short supply in many parts of Japan, cities often expand into mountainous areas, leaving such development vulnerable to landslides.

About 240 mm of rain fell in the area in the 24 hours up to Wednesday morning, record-breaking levels equivalent to a month's worth of rain in a usual August, Japan's Meteorological Agency said.

Asphalt roads crumbled under the force of one landslide, while streams of mud cut swathes through neighbourhoods, turning houses into piles of twisted wreckage.

Prime Minister Shinzo Abe cut short his summer vacation to head back to Tokyo as authorities asked for help from the military for rescue efforts.