Dozens trapped as floods sweep Japan

Dozens of people were trapped in buildings and several were missing after powerful floods ripped through parts of Japan today, in the wake of torrential rains.

Military helicopters plucked stranded residents from roofs after waters surged over a wide area when a river burst its banks, swamping a city of 65,000 people.

Dramatic aerial footage showed whole houses being swept away by raging torrents, in scenes eerily reminiscent of the devastating tsunami that crushed Japan's northeast coast in 2011.

One man was seen clutching a concrete post as waters swirled around him.

No one had so far been confirmed dead, but there were reports of people missing, including in landslides that buried buildings.

Television pictures from Joso, a small city north of Tokyo, showed desperate residents waving towels as they stood on balconies trying to summon assistance.

"Please continue to ask for help. Please don't give up hope," an anchorman for public broadcaster NHK told trapped viewers.

More than 100,000 people were ordered to leave their homes after a huge swathe of northeast Japan was battered by torrential rain, with up to 60 centimetres (two feet) falling in some places.

"I've never seen the Kinugawa river burst its banks," 63-year-old Joso resident Akira Yoshihara told AFP.

"My house is on higher ground but I'm worried the water may reach it tonight."

The floods brought confusion, and several people are reportedly missing -- including one in Joso -- but authorities said they were unable to confirm the total number unaccounted for.

By early evening, police, firefighters and troops had rescued at least 260 people from houses in Joso City and surrounding areas, Jiji Press news agency said.

But an estimated 200 people were still trapped in their homes or other buildings, including a day care centre and a seniors' home, Jiji said.

Joso is about 60 kilometres outside Tokyo, which has also been hit by localised flooding.

The rains came in the wake of Typhoon Etau, which smashed through Japan on Wednesday, bringing strong winds and causing travel chaos.

Flooding complicated a contaminated water problem at the crippled Fukushima nuclear plant, where the site's drainage pumps were overwhelmed, sending radiation-tainted water into the ocean, a spokesman for operator Tokyo Electric Power said.

"This is a scale of downpour that we have not experienced before," forecaster Takuya Deshimaru told an emergency press conference.

Prime Minister Shinzo Abe said the government was on high alert, and vowed to put the "highest priority" on saving people's lives.