At least five people have died and another remains missing after record-breaking rainfall hit eastern Japan, flooding roads and homes, disrupting rail services and leaving thousands stranded overnight at Narita airport.

The heaviest rain was reported in Chiba prefecture, east of Tokyo, where authorities issued their highest-level warning for the first time. The Japan Meteorological Agency (JMA) later downgraded the warning as conditions eased on Friday morning, but forecast more rain later in the day.

Record rainfall triggers emergency warnings

The rainfall was described by JMA officials as unprecedented, with more than 30 centimetres of rain recorded in 24 hours at a Chiba weather station.

Chiba City recorded 11.5 centimetres of rain in just one hour, while Sakura recorded 9.7 centimetres in an hour. Both figures were records.

JMA forecast official Takuya Hosomi said during a televised emergency news conference that unprecedented levels of rainfall had been recorded.

The rainfall triggered landslide warnings, flooding and widespread power outages. The JMA's highest-level heavy rain warnings covering more than a dozen municipalities in Chiba were downgraded to level four by Friday morning.

Scientists have said human-driven climate change is making extreme weather events more frequent, prolonged and intense.

Five confirmed dead, one missing

Chiba officials said five people had been confirmed dead by Friday afternoon, while another person was missing.

The victims included a man in his 60s or 70s who was found floating on a flooded road in Ichikawa city and a 66-year-old woman who was trapped in a submerged car in Sakura city.

Two other people were presumed dead after being pulled from flooded cars in Kashiwa city and Chiba. Another victim was found collapsed on a street, according to reports.

Japan's public broadcaster NHK had earlier reported that as many as eight people may have died.

Homes and roads submerged

More than 170 homes were flooded across Chiba prefecture, while nearly 90 others had water seeping in below floor level.

Daylight footage showed homes and cars surrounded by muddy floodwater, with sections of railway tracks also inundated.

In rural parts of Chiba, more than a dozen cars were abandoned on low-lying roads, an AFP journalist reported. Images from Kashiwa showed vehicles sitting in water up to their headlights.

More than 300 people were staying in public shelters by Friday afternoon. Hundreds of others spent the night at the Chiba prefectural government building and other shelters.

A woman in Ichikawa told NHK that the flooding had happened suddenly.

"This happened all of a sudden, so it was a complete surprise. The power is out, so I'm worried whether I can open my business in the next couple of days," she said.

Thousands stranded at Narita airport

About 6,000 people were stranded overnight at Narita airport, one of the two main airports serving Tokyo, after transport services were disrupted.

Passengers slept across the terminal, with sleeping bags distributed as long queues formed at check-in areas. Social media users said airport staff handed out water, food and sleeping bags.

A traveller from the United Kingdom, who gave his name as Philip, told AFP that his journey to the airport had taken nine hours, including a taxi ride.

Holding a sleeping bag, he said he had "just been trying to get some sleep and watching Netflix".

"Just get on with it, really. Nothing else you can do," he said.

Another stranded passenger told NHK: "I never thought I'd spend the night at the airport, but it is what it is and we are learning."

"We're never going to forget this trip," the passenger said.

Rail services disrupted across Chiba

Train services to and from Narita were cancelled or suspended until Friday afternoon, leaving hundreds of passengers stranded at Chiba station.

Thousands of people at Narita gradually left the airport after train services partially resumed on Friday morning, although delays continued.

Flights in and out of Narita were operating normally on Friday. Many passengers were travelling home at the end of Japan's Bon Buddhist holiday week.

The Ground Self-Defense Force also sent buses to Soga train station to pick up about 4,000 people who had spent the night there. The operation was completed later on Friday.

The station serves a local rail route that also provides access to Tokyo Disney. The theme park reported no damage.

Power cuts affect tens of thousands

The heavy rain and flooding also caused widespread power outages across Chiba.

About 18,000 homes remained without electricity on Friday morning, according to TEPCO Power Grid, down from around 68,000 earlier.

At one point, the prefecture's disaster management office said close to 26,000 households were without power.

Authorities continued to monitor flooding and rainfall as further rain was forecast for Friday.