At least 39 people have been killed and another seven are still missing after a huge landslide engulfed homes in western Japan.

Steep hillsides saturated by weeks of rain gave way in Hiroshima in the early hours of Wednesday, and engulfed the north of the city.

Houses were buried or swept off their foundations as torrents of water, mud and rocks tore through heavily populated valleys.

The NHK news agency says the death toll has reached 39 while 7 others are unaccounted for.

The number of dead rose rapidly from an initial toll of four, although emergency services said it was too early to tell exactly how many people had lost their lives.

"I woke up in the middle of the night and the corridor leading to the living room of my house was already flooded," one resident told the BBC.

"I heard the sound of water coming in, and then the water from the river rushed into my house, so I just took the car and rushed out."

Another resident told Fuji TV: "There was rain and thunder all night, beating down so hard I was scared to go outside. Great big drops. I've never seen anything like this."

Among the victims were a 77-year-old woman, a two-year-old boy and his 11-year-old brother, with the children dug out of the debris after their house was struck as they slept.

Helicopters clattered over the scene, lifting out survivors, as rescue workers searched through mud and piles of stones in residential areas about 5 kilometres from the city centre.

Aerial footage showed several houses buried by sludge, their wooden frames splintered by the weight of the mud.

Torrents of brown water ran off mountains behind the homes and through the wrecked buildings, hampering rescuers' efforts as they searched for anyone still trapped.

Emergency workers were seen climbing up to the second floor and roofs of half-collapsed houses - some of which were floating - in a bid to reach any survivors.

Rescue workers walk with a stretcher after a massive landslide swept through a residential area at Asaminami ward in Hiroshima. ( Reuters/Kyodo )

Pictures showed there had been at least five different landslides, some having uprooted trees and carried rocks down the hillside.

One man, gesturing to the mud-covered remains of a house, told NHK: "My house is over there, flattened."

Pointing elsewhere, he said: "A leg was seen [sticking out of the mud] and they are trying to confirm if the person is alive. The first thing we have to do is to help that person."

Japanese troops were deployed in response to a request from the local government.

Prime minister Shinzo Abe said there would be a sizeable response.

"I have ordered [government officials] to carry out the rescue operation in an integrated manner, aware of the possibility of further rain," he said in Tokyo.

"I also ordered them to raise the number of Self-Defence Force personnel to several hundred in order to strengthen rescue operations," he said, adding he would be sending one of his ministers to the site.

Japan's weather agency warned more heavy rain was on the way to the area, raising the risk of further landslides in places where tonnes of mud have already been displaced.

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

The archipelago has been battered in recent weeks by unusually heavy rain that has sparked a number of smaller landslides and several floods, some of which have claimed lives.

Despite widespread concreting to shore up hillsides, mountainous and densely populated Japan is prone to this kind of disaster.

In October last year, dozens of people were killed when the torrential rains of a passing typhoon triggered large landslides on the island of Oshima, south of Tokyo.

Landslides killed 31 people in Hiroshima in 1999.

An aerial view shows the damage caused by the landslide ( AFP )

AFP/Reuters