Typhoon Shanshan weakened to a tropical storm on Friday, but was still dumping heavy rains as it slowly churned through Japan, triggering transport havoc and landslide warnings, with up to six people killed.

The storm, which at landfall was one of the strongest to hit Japan in decades, pummelled Kyushu island on Thursday with gusts of up to 252 kilometres per hour.

Even before making landfall, a landslide caused by the heavy rains preceding the storm killed three members of the same family late on Tuesday in Aichi prefecture.

Government spokesman Yoshimasa Hayashi on Friday confirmed reports of one additional death, but said that "the relation to the typhoon was being studied".

Two more were feared dead and two others were missing, he added.

Eight people were seriously hurt and 70 others had light injuries, Hayashi said, with many injured by broken glass after the typhoon smashed windows and ripped tiles off roofs on Thursday.

Almost 200 buildings were damaged.

Typhoons in the region have been forming closer to coastlines, intensifying more rapidly and lasting longer over land due to climate change, according to a study released in July.

A rapid attribution analysis issued on Friday by Imperial College London, using peer-reviewed methodology, calculated that Typhoon Shanshan's winds were made 26 percent more likely by a warming planet.

"Without phasing out fossil fuels, the root cause of climate change, typhoons will bring even greater devastation to Japan," said Ralf Toumi, director of the Grantham Institute at Imperial.

The coming storm prompted Japanese authorities to issue their highest alert in several areas, with more than five million people advised to evacuate their homes.

On Friday the Japan Meteorological Agency issued alerts for possible landslides in many parts of Kyushu, and as far away as Shizuoka on the main island of Honshu, the Tokyo region and nearby Kanagawa. (AFP)