TOKYO (AFP) - The death toll from heavy rains and flooding in southern Japan has risen to 15, a government official said on Saturday (July 8), as rescuers continued work to evacuate isolated survivors.

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 rain to search for missing and stranded people, with more than 500 believed to still be cut off, according to public broadcaster NHK.

Chief Cabinet Secretary Yoshihide Suga told reporters early Saturday that the death toll "rose to 15" and 16 people were still missing.

Local governments announced Friday that a total of eight people have died.

"The region is expected to see a localised heavy rain today," Suga said, advising local residents to gather information from the governments and media.

Separately, three women were found dead at the sea facing Saga prefecture, near the hard-hit Fukuoka prefecture city of Asakura, NHK said, adding that they were likely the victims of the heavy rain.

Deep mud and soaked ground on steep hillsides as well as knocked-out bridges have hampered rescue work.

TV footage has shown rescuers strapping people to cables to be lifted up to helicopters and ferried to safety in evacuation shelters.

Broken trees, their bark stripped away, littered the scenic and verdant landscape like broken matchsticks as thick clouds hovered over green mountains.

Aerial TV footage on Friday showed desperate residents of one isolated area using uprooted trees to spell out "SOS" for rescuers to see.

Fallen trees smashed into houses in Asakura, which saw more than 50cm of rain in a 12-hour period to Wednesday night.

"My parents are still trapped with 16 other people in the Kurogawa area and I have absolutely no information about the situation there," Asakura resident Kayoko Ishibashi told AFP Friday, referring to a district in the city.

"So I can only wait here in the hope that they will be rescued by helicopter," she added. "It's the same for everyone here." The government has dispatched some 12,000 police, military, firefighters and coast guard personnel for rescue operations.

Japan's Imperial Household Agency said that out of consideration for the disaster-hit region, Emperor Akihito's eldest granddaughter Princess Mako and her fiancé decided to postpone Saturday's scheduled formal announcement of their engagement.