Japan on Thursday called off the search until next year for seven hikers still missing on a volcano that violently erupted last month, saying freezing conditions had made the operation impossible.

Troops, firefighters and police have been scouring Mount Ontake in central Japan for the missing climbers since the volcano erupted without warning on September 27, killing at least 56 people in Japan's deadliest eruption for almost 90 years.

But officials said rescuers were now battling brutal weather conditions as well as the thick, sticky ash -- up to half a metre (20 inches) deep -- that has made the search operation so difficult.

Some of the dead have been found half-buried, leading to fears others may be entombed.

"We have decided to call off the operation for the rest of this year," said an official of the crisis management office of Nagano prefecture, where the volcano sits.

"Snowfalls and freezing on the slope ahead of this year's full-fledged winter were already adding to the tough conditions up there," the official added.

"We won't carry out the large-scale operation, but we may begin a small-sized one next year when spring comes."

Rescuers were using hand-held metal detectors and sticking poles into the ground in a bid to find victims possibly buried under the ash.

The rescue work was earlier stalled by heavy rain triggered by typhoons.

Mount Ontake was packed with walkers out to witness the region's spectacular autumn colours when it burst angrily to life last month.

Autopsies have revealed that the walkers, many of whom had been enjoying lunch at the peak in the sunshine, died largely from injuries caused by stones hurled out in the initial explosive eruption.

The volcano continues to belch steam and poisonous fumes, making a section near the crater inaccessible.