NHK images showed large areas of the city of Hitoyoshi in Kumamoto flooded in muddy waters that gushed from the Kuma River. Many cars were submerged to the windows.
Mudslides hit houses and floodwaters carried uprooted tree trunks. Several people were standing on top of a convenience store while waiting for rescuers.
Kumamoto Governor Ikuo Kabashima later told reporters that 14 residents of a flooded elderly care home in the village of Kuma were presumed dead after being found during rescue operations. He said three other elderly residents were suffering from hypothermia.
They were among sixty residents of the nursing home on the banks of the Senjuen River, where floodwaters and mud gushed out, blocking the residents, NHK said. The Japanese Self Defense Force said it had sent troops to join the rescue efforts at the site.
In Tsunagimachi district, two of the three people buried under the mudslides were removed without vital signs, said Takafumi Kobori, head of prefectural crisis management in Kumamoto. Rescuers were still looking for the third person.
In another severely flooded town, Ashikita, six people are missing and a seventh has been seriously injured, said Kumamoto officials.
In the mountainous village of Kuma, residents stranded at home were transported by rescue helicopter. In the city of Hitoyoshi, rescuers transported some residents in a boat.
The floods also cut power and communication lines. According to the Kyushu Electric Power Co., about 8,000 homes in Kumamoto and neighboring Kagoshima were without electricity.
Prime Minister Shinzo Abe has set up a task force and said that up to 10,000 defense soldiers were mobilized for rescue operations.
The Japan Meteorological Agency issued warnings of extraordinary rain in parts of Kumamoto, about 1,000 kilometers southwest of Tokyo, but then degraded it as precipitation – estimated at 10 centimeters per hour – decreased.