Raging rivers overflowing with water and mud have devastated swathes of Kyushu -- the southernmost of Japan's four main islands -- after heavy rainfall, sweeping away roads and houses and destroying schools.
Thousands of rescuers have been fighting through thick mud and battling the rain to search for missing and stranded people, with more than 1,100 believed to be cut off according to public broadcaster NHK.
The government said Friday that six had been killed, while 22 remain unaccounted for.
NHK footage showed rescuers removing the body of a victim from a damaged home and heavy machines moving rocks and dirt to clean roads.
A number of fallen trees were shown smashed into houses in the hard-hit Fukuoka prefecture city of Asakura, which saw more than 50 centimetres (almost 20 inches) of rain in a 12-hour period to Wednesday night.
Vehicles could also be seen overturned or buried in mud and reinforced river banks destroyed by raging water. Military trucks and rescue vehicles competed for space on the city's streets.
NHK said local authorities were dispatching helicopters to pluck people out of isolation, showing footage of stranded elderly residents being rescued.
It added that local authorities were rushing to restore access to regions cut off by the landslides and floods.
In the nearby city of Hita in neighbouring Oita prefecture, Masayoshi Arakawa said he had experienced heavy rains in the past but this year's deluge was unexpected.
"A few years ago I had no problems so I thought that's how it would go again and so I decided to spend the night at my house last night," he told AFP late Thursday at an evacuation shelter.
"But when I went out to see how it was outside, I became frightened."
The government's top spokesman, Chief Cabinet Secretary Yoshihide Suga, told a press conference that some 12,000 police, military, firefighters and coast guard personnel were taking part in rescue operations.
As of Thursday they had recovered 522 people, and a further 15 on Friday.
"Heavy rain is forecast to continue intermittently," Suga said, while calling for continued vigilance.
"I would like people in the disaster zone to pay full attention to evacuation information."
In Hita, Katsuko Noda was also among those forced to flee her home.
"A neighbour came to see me and told me that there was a landslide and that the water could reach us, so I took my bag without further delay and I came to take refuge here," Katsuko Noda told AFP at the evacuation shelter.
Heavy rain and landslide warnings remained in place on Friday.
The Japan Meteorological Agency said it expected as much as 25 centimetres of torrential downpours in the 24 hours until Saturday morning.
"Due to sustained heavy rainfall, the area is seeing increased risk of landslides," the weather agency said, adding some areas of northern Kyushu had experienced "unprecedented" rainfall.
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