Thousands of rescuers have arrived in a deluged city north of Tokyo to help evacuate hundreds of trapped residents and search for 12 people missing after torrential rains triggered massive flooding.
Heavy rain continued early on Friday, threatening to worsen conditions in the wake of Typhoon Etau, which smashed through the country earlier this week bringing strong winds and causing travel chaos.
One person is so far confirmed to have died in the flooding.
Some 2,000 troops, police and firefighters were dispatched on Friday morning to flooded areas where rescuers had worked through the night, public broadcaster NHK said.
Television footage at daybreak from Joso, a small city of 65,000 in Ibaraki prefecture and the worst affected area, showed dozens of residents getting into military boats from a shopping mall cut off by the floods, while helicopters plucked others to safety.
Images showed residents walking in knee-deep water near shelters in the city, some 60 kilometres outside Tokyo, which has also been hit by localised flooding.
Some 6,500 homes are believed to be in the area affected by the floods, triggered when a levee on the Kinugawa river gave way on Thursday.
"At least 12 people are still unaccounted for as of Friday morning, while seven others were injured," said Hiroaki Tachi, an official of Ibaraki prefecture.
"We are trying our best to rescue people as soon as possible, while still asking our residents to stay vigilant for now," the official said. In Kanuma, north of Joso, a 63-year-old woman was killed after being engulfed with landslides triggered by heavy rain, a local official said.
On Thursday, dramatic aerial footage showed whole houses being swept away by raging torrents.
An estimated 690 people were awaiting rescue on Thursday night, the National Police Agency said, according to NHK. It was not immediately clear how many people were still trapped by Friday morning.
More than 100,000 people had been ordered to leave their homes on Thursday after a huge swathe of northeast Japan was battered by torrential rain.