At least four hikers have been pronounced dead and the fate of 27 others are due to be confirmed after the group was caught near the peak of a Japanese volcano that erupted without warning.

Rescue workers battling rocketing levels of sulphurous gas on Sunday found 31 people in "cardiac arrest" near the summit of 3,067m Mount Ontake, which erupted on Saturday.

The term is usually applied before doctors can certify death.

Four of those found were brought down the volcano on Sunday where they were confirmed dead, broadcaster NHK reported.

"According to the police, four people out of the 31 people who had been found in cardiac arrest near the peak were confirmed dead. All of them were men," the channel said.

The news appeared to confirm fears that there was little hope for those still on the volcano.

Rescue efforts were called off in mid-afternoon on Sunday because the environment was becoming too dangerous for emergency workers, an official at the Nagano prefectural government told the AFP news agency.

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

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

About 550 soldiers, police and firefighters took part in a major operation to reach those stranded on the volcano since it erupted.

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

Hikers who descended from the volcano reported stones raining down and hot ashes filling the air.

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

The meteorological agency forecast further eruptions, warning that volcanic debris may settle within 4km of the peak.

Source: Agencies