Japan’s Meteorological Agency urged nearby residents to stay away from Mt. Kusatsu Shirane after it detected what it said was “slight volcanic activity.”
“Today an eruption occurred” at the mountain, agency official Makoto Saito told a news conference.
He warned that the volcano could still spew more rocks and ash, and said there was a risk of further avalanches.
A local fire department official told AFP that 10 people had been injured 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 a ski resort in Gunma, northwest of Tokyo.
The official said an earlier report that one person was missing was not accurate.
The defense ministry said six infantry personnel who had been training on the mountain were also hit by the avalanche but had been rescued.
“They are injured but their injuries are not life-threatening,” a ministry spokesman said.
Footage broadcast on Japanese television showed thick black smoke interspersed with falling rocks sliding down the snow-covered side of the volcano toward a ski slope.
The falling rocks kicked clouds of snow into the air and they made impact.
“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.
“About 100 people have evacuated,” he said.