Sixteen people remain unaccounted for on a still-smouldering Japanese volcano that erupted at the weekend, officials said Friday, as a typhoon looming off the coast threatened to further delay the stalled recovery operation.

It is the first time an official figure has been put on the number of people missing since the volcano roared to life, and comes on top of the 47 people whose bodies have been removed from the peak of Mount Ontake.

"We arrived at the figure by checking climbing reports and cars parked nearby as well as listening to climbers and families," an official from Nagano Prefecture told reporters.

The volcano continues to belch steam and poisonous fumes, making a section near the crater inaccessible, despite the presence of hundreds of well-equipped troops, police and firefighters.

Rescue workers have spoken of up to half a metre (20 inches) of thick, sticky ash smothering the slopes, with some of the dead found half-buried, leading to fears others may be entombed.

Television footage showed grey cinder-filled water gushing down a swollen river near the base of the mountain, where rescuers waited.

Heavy rain forced rescuers to abandon their search of the mountain on Friday, while a gathering typhoon looked set to batter Japan over the coming days, a further reminder of the country's vulnerability to the wrath of nature.

"We have decided to call off the day's operation as rain continued near the mountain," said an official of the crisis-management office of Nagano.

"We wanted to speed up rescue operations as we are aware that families and friends are desperately seeking information, but we are concerned about secondary accidents due to the bad weather," the official said.

"We are now also concerned about a typhoon that is forecast to approach the Japanese archipelago and affect large areas, including our region," the official added.

Typhoon Phanfone is predicted to slam into Japan with strong winds and high waves this weekend or later, the meteorological agency said.

Packing gusts of up to 252 kilometres (156 miles) per hour, Phanfone, one of this year's strongest typhoons, moved northwest in the Pacific toward Japan's southwest. It was forecast to make landfall on Sunday.

That could force the recovery to be put off for days, heaping anguish on the families of those still missing, who are gathered at the foot of the volcano.

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.

Mount Ontake was packed with walkers 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.

In Tokyo, Prime Minister Shinzo Abe told parliament: "We want to speed up taking measures to strengthen the (volcano) monitoring system and improve procedures in providing information for climbers."