'Unprecedented' rains in Japan kill four people

At least four people have died and five others are wounded in eastern Japan as record rainfall pummelled the Chiba prefecture near the capital Tokyo.

The rains, which authorities have described as "unprecedented", cut power to more than 20,000 households while 7,000 travellers were stranded overnight at Tokyo's Narita airport.

Local authorities issued a level five emergency warning, its highest alert, on Thursday. This was downgraded to a level four by Friday morning, but they continue to warn of landslide and flooding risks.

The deluge comes after a series of storms swept through Japan and neighbouring countries this week, in what's been forecast to be an unusually active typhoon season.

Japan's Meteorological Agency said the most recent deluge was brought by warm, humid air flowing into the country's east that combined with cold air above to make atmospheric conditions extremely unstable.

"This case was an extremely unusual situation, even by Japan's historical weather standards," Chiba Governor Toshihito Kumagai told reporters on Friday morning.

"I have responded to many disasters in the past, but I have never experienced a case like this."

Chiba, which sits right next to the capital city of Tokyo, has logged more than 100mm of rain per hour since Thursday afternoon, authorities say.