A Japanese military officer has died and 11 others injured when a volcano erupted and a subsequent avalanche trapped skiers at a mountain resort in central Japan.

Key points: Japanese media report at least 12 people injured

Japanese media report at least 12 people injured Unclear whether avalanche caused by volcanic activity, both occurred nearly simultaneously

Unclear whether avalanche caused by volcanic activity, both occurred nearly simultaneously One person trapped in avalanche before being dug out by rescuers

Six skiers, including the man who died, were among 30 Japanese soldiers from the Ground Self Defence Force (SDF) who were participating in ski training when they were buried by the avalanche, fire department and defence officials said.

All of the people were rescued, but a 49-year-old male soldier who suffered broken bones later died, an official at the regional military training camp said.

At least 11 people skiing on the slopes of the mountain were injured by flying rocks during the eruption and avalanche.

Five of the civilian skiers suffered serious injuries such as broken bones, but none of the injuries were life-threatening, Wataru Tatsukawa, a regional fire department official, said.

Defense Minister Itsunori Onodera told reporters that seven people trapped by the avalanche at the ski resort on the side of the 2,171 metre-high active volcano were all rescued.

Town officials said they believe everybody has been accounted for.

Only the crater had been off-limits because of low-level volcanic activity before Mount Kusatsu-Shirane erupted with rocks and ash about 10:00am (local time).

The Japan Meteorological Agency has since banned access to the mountain, and a large swath of the snow-covered volcano was covered by dark grey ash later in the day.

One person was trapped in the avalanche for some time before being dug out by rescuers. ( AP/Kyodo News: Suo Takekuma )

The agency also said the eruption and the avalanche could not be linked immediately.

Snow conditions and seismic activity are some of the potential causes of an avalanche.

'There was this huge boom'

The ski area's gondola was suspended after the eruption and about 80 skiers took refuge at a gondola station at the top of the ski slope, according to the fire department official.

A military helicopter airlifted the first group of eight skiers to safer ground. The rescued skiers, wearing helmets, were handed a bottle of tea each and escorted into a cabin.

The slopes were filled with volcanic ash and rocks and rescuers were considering whether using snowmobiles to reach the stranded skiers was safe.

"There was this huge boom, and a big plume of totally black smoke rose up," one skier told NHK.

"I had absolutely no idea what had happened."

An earlier report that one or two people had been injured by shattered glass when rocks struck a gondola had not been confirmed.

A rest house at the resort was hit by volcanic rocks, but the extent of damage was not known, resort official Yasuaki Morita said.

The last time Kusatsu-Shirane erupted was in 1983.

Japan sits on the Pacific "Ring of Fire" and is prone to earthquakes and volcanic eruptions.

An eruption of Mount Ontake in 2014 killed about 60 people. Several other Japanese volcanoes are considered unstable with small eruptions in recent years.

Reuters/ABC