Dozens of people were left stranded on rooftops and more than 75,000 people were urged to evacuate from the Kumamoto and Kagoshima prefectures in western Japan, about 1,000 kilometers (600 miles) southwest of Tokyo, following the rain.
TV broadcasts showed footage of houses swept away by mudslides in the city of Ashikita, and cars submerged to their windows and residential areas swamped beneath muddy water in the city of Hitoyoshi.
Authorities received over 100 rescue calls but could not respond to all of them immediately, NHK - Japan's national broadcaster - said.
Fourteen of those known to have died were reportedly residents in the same care home.
Japan’s Meteorological Agency issued a Level 5 alert for floods and landslides, its highest warning, for rain “never seen” before — the first time it has ever issued such an alert for the two prefectures, the Japan Times reports.
At its peak the rain was estimated to be falling at 100mm (4 inches) per hour at times and in some parts of the country record rainfall was measured with up to 381mm (15 inches) over six hours.
The government said it was sending some 10,000 Defence Force personnel to the region.
Prime Minister Shinzo Abe said the government would work closely with the prefectural governments to assess damage and aid recovery “while putting lives as their highest priority.”
He added: “There’s a possibility that heavy rain will continuously fall on the Kyushu region through (Sunday), so we must maintain our vigilance at the highest level.”
Today, southern Japan recorded its heaviest rainstorm on record: 15 inches in just 6 hours.
We are in a climate emergency. https://t.co/5V993bcBT7— Eric Holthaus (@EricHolthaus)July 4, 2020
One woman told NHK: “It wasn't like normal rain. I honestly never imagined it could be so powerful.“
A man said: “The smell of the river and the muddy water was intense. I could feel the vibrations from the churning water through the road next to the riverbank.”
According to AP, in the Tsunagimachi district, two of three people buried underneath mudslides were pulled out without vital signs. Rescuers were still searching for the third person.
In Ashikita, six people were unaccounted for and a seventh was seriously injured, Kumamoto officials said.
In the mountainous village of Kuma, residents stranded at their homes were being airlifted by a rescue helicopter. In Hitoyoshi city, rescuers transported some residents in a boat.
Flooding also cut off power and communication lines. About 8,000 homes in Kumamoto and neighboring Kagoshima were without electricity, according to the Kyushu Electric Power Co.
Additional reporting by AP.