More than 30 hikers were feared dead after being found Sunday near the peak of a Japanese volcano which erupted without warning, spewing ash, rocks and steam.

Rescue workers battling rocketing levels of sulphurous gas found them in "cardiac arrest" near the summit of 3,067-metre (10,121-foot) Mount Ontake, which erupted around noon on Saturday, police and local officials said.

The term is usually applied before doctors can certify death.

Local media put the exact figure at 31.

"We have confirmed that more than 30 individuals in cardiac arrest have been found near the summit," a Nagano prefecture police spokesman told AFP without elaborating.

Rescuers, who had to call off the search mid-afternoon Sunday, were bringing down four of them, apparently without administering any immediate medical treatment, said an emergency official at the Nagano prefectural government.

"The rescue team suspended their operation because of the increasing concentration of sulphurous gas in the area," the official told AFP.

Firefighters have separately confirmed a total 30 people with injuries, including one serious case, he said, adding that the number could still change.

Some 550 soldiers, police and firefighters took part in a major operation to try to save dozens of hikers feared stranded on the volcano since it erupted into a sunny autumn sky during a busy weekend for tourists and hikers.

A suffocating blanket of ash up to 20 centimetres (eight inches) deep covered a large area of the volcano, and had forced up to 150 to seek refuge in mountaintop shelters at one point.

Local officials believe 45 to 49 people sheltered overnight in cabins on the mountain, although details remained unclear.

The mountain is popular among walkers, particularly in late September when the turning of the autumn leaves makes for dramatic scenery.

On Sunday, columns of thick white steam were still rising from Mount Ontake, feathering out into the clear blue sky.

- Scenes of horror -

Hikers who descended from the volcano reported scenes of horror, with stones raining down and hot ashes filling the air.

Video footage shot inside a cabin, taken shortly after the eruption and shown on NHK, revealed the screams of terrified hikers as rocks thundered against the roof and walls.

Grey, ash-filled air could be seen rolling against the window before it thickened into darkness, blocking out the sunlight and leaving just the soundtrack of debris pounding on the structure.

A group of 25 hikers -- including a schoolchild -- who spent the night in a cabin, were able to climb down Sunday to reach the start of one a trail.

A middle-age man who was among the group said they had been near the summit.

"People panicked," he told NHK, his face smudged with ashes. "Honestly, I am glad I was able to come back alive."

Emergency helicopters rescued seven people, including two who were able to wave at a Self Defence Force helicopter.

"The helicopter flew over there very early in the morning to survey the condition. Then it found the two people waving at it," said a spokesman at Otaki village in Nagano prefecture.

"Originally, the rescuers thought it might be difficult to go near them because ash could rise (and damage the helicopter), but the conditions were better than they believed and they were able to rescue the two people," he told AFP.

Some 230 hikers were able to make it to safety soon after the eruption.

Many were spattered with the mud that had been spewed out by some parts of the mountain.

Television footage showed a line of rescue workers in orange uniforms or green camouflage walking through what looked like a moonscape.

Among the injuries were those caused by flying rocks and internal burns from inhaling volcanic fumes, the Yomiuri Shimbun said.

The meteorological agency forecast further eruptions, warning that volcanic debris may settle within four kilometres (2.5 miles) of the peak.

The agency also restricted access to the mountain, while calling on local residents to remain alert since an eruption could shatter windows miles away.

The last significant eruption of Mount Ontake, which straddles Nagano and Gifu prefectures in the centre of the country, was in 1979 when it expelled more than 200,000 tonnes of ash, according to local media.

There were more moderate eruptions in March 2007 and in May 1991.