The death toll continues to rise, from unprecedented rainfall and flooding in western Japan.

More than 85 people have died and dozens more are missing.

Millions more have been forced from their homes, with continued heavy rain expected to cause further flooding and landslides.

CNN's Kaori Enjoji says Japan has never experienced rainfall like it.

"The Prime Minister himself said this is a historic disaster for Japan, even for a nation that is used to natural disasters like earthquakes."

Television footage showed bridges and cars washed away by raging rivers and floodwaters, with people perched on the roofs of their homes, surrounded by water and awaiting rescue.

Japan's Meteorological Agency had issued rare "emergency warnings" against landslides, rising rivers, strong winds and lightning strikes caused by what it called "historic" rains in 23 prefectures across the western and central parts of the country. The rains began last week.

Such warnings are issued in anticipation of the sort of extreme conditions that occur just once or twice in 50 years, Kyodo News reported.

"There are still many people missing and others in need of help. We are working against time," Prime Minister Shinzo Abe said.

More than 1.6 million people were ordered to evacuate their homes, while 3.1 million were put on high alert and urged to do so. Nevertheless, Kyodo said, many had remained at home.

Chief Cabinet Secretary Yoshihide Suga said 54,000 police officers, firefighters, and members of Japan's Self-Defence Forces and coast guard had been mobilised in the rescue effort, Kyodo reported, with TV footage showing them using boats and helicopters to bring people to safety.

Public broadcaster NHK said flooding and landslides were hindering rescue efforts and repeatedly urged people not to lose hope.

Among the dead was a 3-year-old girl whose home was hit by a landslide in Hiroshima prefecture, Reuters reported.

"It's very painful," said one elderly man watching nearby. "I have a granddaughter the same age. If it were her, I wouldn't be able to stop crying."

Two sisters from an elementary school with just six students on the small island of Nuwa in Ehime prefecture also died, according to Reuters. The younger, a first-grader, was a star and the hope of the depopulated island, the principal told NHK.

The western Hiroshima prefecture was hit the hardest by landslides, which claimed 37 lives, while 21 people died in Ehime, NHK reported.

In August 2014, 77 people died in Hiroshima when torrential rain triggered massive landslides, but one resident told Kyodo that the rains were heavier this time.

Auto manufacturers Mitsubishi and Mazda were forced to halt production at some factories because they could not get parts or did not want to force employees to travel to work in dangerous conditions, Kyodo reported.