Emergency personnel were being deployed from early today following the inundations overnight that caused authorities in Kyushu island and nearby areas to issue emergency evacuation orders to hundreds of thousands of people.
The city of Asakura, Fukuoka prefecture, 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.
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."
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Ouchida said city officials had received reports of people cut off by the waters.
"It is very dark outside now so it's difficult for us to confirm those reports. We are also hearing about houses being swept away and landslides blocking streets," he said.
A total of 6,000 troops, police officers and other emergency personnel will carry out search and rescue missions from early today, Chief Cabinet Secretary Yoshihide Suga said, according to national broadcaster NHK.