Dozens of houses were buried when hillsides collapsed after torrential downpours in Hiroshima, television pictures showed, leaving rescuers to pick through the devastation for any signs of life.
A spokesman for Hiroshima Police told AFP the death toll was still climbing.
"The figures may change as the rescue efforts continue," he warned.
The number of dead had risen rapidly from an initial four, which included a two-year-old boy.
Among the dead was a 53-year-old rescuer, who was killed by a secondary landslide after he had pulled five people to safety, the Fire and Disaster Management Agency said.
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.
"We haven't assessed the full extent (of the disaster) yet," said a spokesman for Hiroshima's fire department earlier.
Rescuers said the victims included a two-year-old child and a 77-year-old woman.
Another emergency services spokesman said several more people were missing although the exact number was not yet confirmed.
There are "several spots where people are supposed to be buried alive, and we still don't know how many people are missing", he told AFP.
Aerial footage showed several houses buried by the 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 to get to those 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.
Footage showed there had been at least five different landslides in the same hillside area, some having uprooted trees and carried rocks down the hillside.
Japanese troops were deployed in response to a request from the local government.
Prime Minister Shinzo Abe said it would be a sizeable deployment.
"I have ordered (government officials) to carry out the rescue operation in an integrated manner, aware of the possibility of further rain," he told reporters in Tokyo.
"I also ordered them to raise the number of Self-Defense Force (military) 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.
