"His lungs were damaged" because of the impact of the rocks, Onodera said, adding that the other seven soldiers had sustained injuries.

The defence ministry had initially said a total of six infantry troops were caught up in the incident.

"Black smoke rose from the top of the mountain and we were told to evacuate inside 30 minutes later," a man who was at the ski resort told public broadcaster NHK.

Around 80 people were stranded for hours at a gondola station at the top of the mountain after a power outage.

Television footage showed military helicopters buzzing overhead as people were airlifted to safety.

Yuko Iguchi, an official from nearby Kusatsu town, told AFP: "All the people stranded at the summit were brought down to the safe zone. We have not received any injured people among them."

Another local official, Yoichi Takai, told AFP that "15 people from Taiwan and four from Britain were among those who were stranded at the peak."

One woman who was trapped at the gondola station but later rescued told Japanese TV: "There was an eruption in front of my eyes. Black and white plumes came towards me. Then the ski slope went completely black."

The area had seen heavy snowfall in the hours leading up to the eruption, making the slopes ripe for avalanches.

A snowboarder told NHK his gondola had stopped suddenly and he saw that other gondolas around him had broken windows and were covered in ash.

"I realised it was an eruption. Forty to 50 minutes later, all the gondolas moved up to the station on the top of the mountain," he said.

Ring of fire

Japan's Meteorological Agency urged people to stay away after it detected what it said was "slight volcanic activity."

Agency official Makoto Saito told reporters the volcano could still spew more rocks and ash, and that there was a risk of further avalanches.

A local fire department official told AFP that 10 people had been hurt in the incident.

"Five of them were seriously injured. We began sending the injured to a hospital," he said.

Among the injured were four people hurt by shattered glass while on a ropeway gondola at the ski resort.

The official said an earlier report that one person was missing in the avalanche was not accurate.

Japan, with scores of active volcanoes, sits on the so-called Pacific "Ring of Fire" where a large proportion of the world's quakes and volcanic eruptions are recorded.

On September 27, 2014, Japan suffered its deadliest eruption in almost 90 years when Mt Ontake, in central Nagano prefecture, burst unexpectedly to life.

An estimated 63 people were killed in the shock eruption, which occurred as the peak was packed with hikers out to see the region's spectacular autumn colours.

According to the Meteorological Agency, this was the first time Mt. Kusatsu Shirane has erupted since 1983.

There are currently 111 active volcanoes in Japan, according to agency officials.