An official says a volcanic eruption near a ski resort in central Japan has injured at least nine people.

Twelve people, including eight soldiers, skiing on the slopes of a volcano near a famous hot spring resort in central Japan were injured on Tuesday by flying rocks during a sudden eruption, officials said. One soldier later died.

The eruption quickly darkened the ski slope at Mount Kusatsu-Shirane with black ash, as volcanic rocks rained down on gondolas and people skiing down the slopes, sending them desperately seeking shelter at a mountaintop station.

The eight soldiers were in a group of 30 who were undergoing ski training and were close to the mountain's peak when they were hit by the volcanic rocks, defence officials said.

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

One of the soldiers, who was slightly injured, said he took refuge in a forest by the ski slope after seeing black smoke and volcanic rocks shooting into the air. When visibility improved about 10 minutes later, he found several fellow soldiers fallen to the ground, Kyodo News reported.

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