One soldier died and 11 others were injured while skiing in central Japan on Tuesday, the result of flying rocks sent airborne during a sudden eruption of a volcano, officials said.

Eight soldiers were among a group of 30 undergoing ski training when they were hit by the volcanic rocks, defence officials said.

The officials had earlier said the injuries were caused by an avalanche, but later corrected that account.

The four civilian skiers did not have life-threatening injuries, Gunma prefectural disaster officials said.

Only the crater had been off-limits because of low-level volcanic activity before Mount Kusatsu-Shirane erupted around 10 a.m. 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.

The agency also said the eruption and the avalanche could not be linked immediately. Snow conditions and seismic activity are some of the possible causes of an avalanche.

5 soldiers seriously injured

Defence Minister Itsunori Onodera said five of the eight soldiers were seriously injured. One of them, a 49-year-old man, later died, said Wataru Tatsukawa, an official at the regional military training camp.

Town officials said everyone on the mountain has been accounted for, and all of about 80 skiers who took refuge at a gondola station at the top of the ski slope were brought down to the foot of the mountain, some by a military helicopter, others by snowmobiles. The rescue operation was finished by the evening.

Japan Volcano

A helicopter of the Japan's Ground Self-Defense Force flies at a ski resort in Kusatsu, central Japan for the rescue operation on Tuesday. (Muneyuki Tomari/Kyodo News via AP)

NHK public television showed the first group of rescued skiers, wearing helmets, being handed a bottle of tea each and escorted into a cabin.

An earlier report that some 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.

Kusatsu-Shirane last erupted 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 and have had small eruptions in recent years.

Japan Volcano

Ashes from Mount Kusatsu-Shirane cover near its summit after its eruption in Kusatsu, Gunma prefecture, central Japan, on Tuesday. (Suo Takekuma/Kyodo News via AP)