The torrential downpours late Thursday triggered landslide warnings and power outages, with forecasters issuing the highest-level heavy rain warning for the Chiba region for the first time.

In Chiba's rural parts, more than a dozen cars were left abandoned on low-lying roads, an AFP journalist saw.

At Narita, sleeping bags piled up as passengers slept all over the terminal, with massive lines forming for check-in across the airport.

A traveler from the United Kingdom who gave his name as Philip told AFP that he had to go through a nine-hour journey, including a taxi ride, before finally arriving at the airport early Friday morning.

With a sleeping bag in hand, he said he has "just been trying to get some sleep and watching Netflix".

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

Those killed included a man in his 60s or 70s found floating in a flooded road and a 66-year-old woman stuck in her submerged car, the Chiba Prefecture Disaster Management Office said.

The third fatality was a man of unknown age found collapsed on the street, and the fourth a person of undetermined gender and age.

Public broadcaster NHK reported as many as eight people may have died.

Meanwhile, close to 26,000 households across Chiba prefecture were without power, the disaster management office said.

A total of 45 houses flooded in cities such as Ichikawa, Kashiwa and Sakura. Another 39 homes saw water seeping in below the floorboards.

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

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

Trains canceled

A Japan Meteorological Agency (JMA) official told a news conference on Thursday that this was "shaping up to be an unprecedented level of heavy rain".

Around 6,000 people were stranded overnight at Narita, one of two main airports serving Tokyo, with many trains to and from there cancelled until Friday afternoon, the Chiba disaster office said.

Users on social media said that airport staff handed out free water, food and sleeping bags.

Hundreds of people were also left stranded in Chiba station due to suspended train services, an AFP journalist saw.

Hundreds spent the night in shelters in Chiba, NHK reported, showing cars in water up to the headlights in Kashiwa city.

"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," a woman in Chiba's Ichikawa city told NHK on Thursday.

The level-5 warnings of heavy rain were all downgraded to level-4 as of Friday morning.

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

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