Meteorologists warned of worse to come and soldiers and rescuers scrambled to evacuate hundreds of thousands of people out of the path of landslides.
Meteorologists warned of worse to come and soldiers and rescuers scrambled to evacuate hundreds of thousands of people out of the path of landslides and the swirling torrents of muddy water.
"We are seeing heavy rains unlike anything experienced before," a Japan Meteorological Agency official told reporters.
"It is an abnormal situation with serious danger looming."
Japan is deploying 7,500 police, rescue personnel and troops in affected areas of the main southern island of Kyushu, while 40 helicopters are on standby for search operations once the weather clears, Chief Cabinet Secretary Yoshihide Suga said Thursday.
"There is a lot of information about people who are unaccounted for," he told a hurriedly arranged press conference.
Those included a child reportedly carried off by a river in spate and a couple who had not been seen since their house was swept away.
Officials in worst affected Fukuoka and Oita prefectures said they knew of 15 people who were missing.
Television footage showed rolling waves from swollen rivers hitting residential areas, tearing up roads and inundating farmland.
The city of Asakura was among the hardest hit with television footage showing floodwaters surging through the streets.
"We are hearing of around six to seven people missing so far. That includes a report of a child seen being swept away," said Naohisa Ouchida, an official with Asakura city's disaster management division.
He was not able to confirm any details of those missing, but said authorities had received "many, many" calls from people in distress.
Some locals took to Twitter to appeal for rescue. The account @ek2eoiNcRUqmnMI posted: "Please help! Most houses around us have been swept away. Seven of us are trapped on the second floor of a factory."
Ouchida said city officials had received reports of people cut off by the waters.
"We are also hearing about houses being swept away and landslides blocking streets," he said.
More than 50 centimetres (20 inches) of rain deluged parts of Kyushu in 12 hours on Wednesday, the tailend of a typhoon that swept through the area earlier in the week.