Rescuers searching the volcano that erupted without warning in Japan found dead hikers wedged between huge rocks and people half buried in ash, it emerged Thursday.

At least 47 people are now known to have died in Japan's worst volcanic disaster in nearly 90 years. But up to 24 are still missing, with fears some could be entombed in the thick, sticky ash that has coated the peak since Saturday's eruption.

Heavy rain forced police, troops and firefighters to abandon their search of Mount Ontake on Thursday, the latest reminder of the ongoing danger posed by the 3,067 metre (10,121-feet) mountain, which is still billowing steam and toxic gas.

Police rescuers found ash 40-50 centimetres (16-20 inches) deep at a shrine on the peak when they first arrived, with some of those killed found collapsed and half-buried, the Mainichi newspaper reported.

Five bodies were stuck in between jagged boulders up to three metres (nine feet) across, which emergency workers had to smash using specialist rock-breaking tools to free them, the paper said.

Another rescuer told the Nikkan Sports daily that moving around on the bed of ash was difficult because of the way it had mixed with steam.

"It sticks like damp concrete," he said.

Rescue workers are having to tread extremely carefully because of worries that the blanket of ash they are walking on could be hiding crevices, or unstable rocks.

Autopsies have revealed that hikers, many of whom had been enjoying lunch at the peak in the autumn sunshine, died largely from injuries caused by stones hurled out in the initial explosive eruption at up to 300 kilometres (190 miles) an hour.

- 'Panic' -

"The bulk of the rocks are estimated to be between the size of a human fist and a head," said Takayuki Kaneko, a vulcanologist at the University of Tokyo's Earthquake Research Institute.

"They fell so densely that some broke up (after hitting other rocks and objects) and scattered," he told public broadcaster NHK.

"I think people there must have had no idea where to run and been plunged into panic," he said.

Among those confirmed dead was a 42-year-old deaf woman, Hiromi Inooka, who was out for the day with her hearing-impaired husband, Tetsuya, 45.

His body has still not been found.

Toshihiro Ono, a fellow member of a drum performance group the couple was involved in said he was shocked to hear they had been caught up in the tragedy.

"Everyday life was hard enough for them because of their disability," he told AFP. "It's difficult to imagine how frightened they must have been up there."

Footage of early rescue efforts released by the Tokyo Fire Department showed a wooden shelter, in which walkers had sought refuge, punctured by sharp rocks.

Inside, futon mattresses lay on the floor, where some of the injured had spent the night before their rescue on Sunday.

But not everyone in the hut had survived; the body of one young man lay sprawled and lifeless in one of the makeshift beds.

Mount Ontake was packed with hikers when it burst angrily to life on Saturday lunchtime. Many would have been there to witness the spectacular colours of the countryside as it turned to autumn.

Hiking is a hugely popular pastime in Japan, with mountain trails promoted by tourism officials who ask walkers to sign in when they begin their trek and sign out again when they finish.

But a local tourism association told the Asahi Shimbun earlier this week that only 10 to 20 percent of hikers do so.

All 47 dead have been identified, but local authorities say based on notifications they have received from family and friends, 24 people remain unaccounted for.

However, they caution the number may include people who did not climb on Saturday or who got down safely.