Dozens of houses were buried when a wall of mud thundered down a hillside in Hiroshima overnight, television pictures showed, with national broadcaster NHK citing local police for the death toll.
Nippon TV gave the same figure. 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.
At least two reports said a 53-year old rescue worker died during the operation when the hillside collapsed again.
It was not clear if this man's death was included in the global toll and there was no immediate confirmation of the reports.
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.
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