Japan struggled to restore utilities and bring relief to the survivors of its worst floods in 36 years, facing health risks from high temperatures and a lack of water, while rescuers kept up an increasingly grim search for victims.

Torrential rains triggered floods and landslides in western Japan last week, killing at least 155 people and prompting Prime Minister Shinzo Abe to cancel an overseas trip to cope with the disaster, which at one point forced several million from their homes.

Power has been restored to all but 3,500 customers, but more than 200,000 remain without water under scorching sun, with temperatures set to hit 33C amid high humidity in some of the hardest-hit areas, such as the city of Kurashiki.

"There have been requests for setting up air-conditioners due to rising temperatures above 30C today, and at the same time we need to restore lifelines," Finance Minister Taro Aso told reporters after a cabinet meeting.

Roads coated in dried mud in the Mabi district of Kurashiki threw up clouds of dust when rescue vehicles or cars drove by. Survivors spoke of narrow escapes.

"It was close. If we had been five minutes later, we would not have made it," said Yusuke Suwa, who fled by car with his wife early on Saturday when an evacuation order came after midnight.

"It was dark and we could not see clearly what was happening, although we knew water was running outside. We did not realise it was becoming such a big deal."

A quarter of flood-prone Mabi, sandwiched between two rivers, was inundated after one of the area's levees crumbled under the force of last week's torrents.

The death toll from the floods rose to at least 155, with 63 missing, public broadcaster NHK said.

The government has set aside 70 billion yen (€536m) in infrastructure funds to respond to disasters, with 350 billion yen (€2.6bn) in reserve, Mr Aso said, adding that an extra budget would be considered if needed.

"When necessary amounts firm up ... we would consider an extra budget later on if these funds prove insufficient."

Japan monitors weather conditions and issues warnings early, but its dense population means every bit of usable land, including potential flood plains, is built on in the mostly mountainous country, leaving it prone to disasters.

A new evacuation order has been issued in one part of the western prefecture of Hiroshima, after a river blocked by debris overflowed its banks.

There was no word on the numbers affected, but the area has already been inundated once.