Rocks spat out by a volcanic eruption rained down on skiers at a mountain resort in central Japan, according to television reports, and one skier was missing following an avalanche that occurred shortly after the volcano exploded.

Three others caught in the avalanche were rescued, Japanese media said, and more searchers were heading to the site, near the hot springs resort town of Kusatsu, to look for the missing skier.

It was unclear whether the avalanche was caused by the volcanic activity.

Kusatsu-Shirane, a 2,160-metre volcano, appeared to have erupted on Tuesday morning, the Japanese Meteorological Agency said.

"Based on various measurements, we can say that the mountain appears to have erupted, but we are still trying to confirm facts on the ground," an agency official said. The warning level for the peak was raised to 3, warning people not to climb the mountain.

Camera footage taken at the resort showed what looked like rocks flying through the air, along with a wave of black smoke.

Four people riding a gondola on the slopes were injured by broken glass, chief cabinet secretary Yoshihide Suga told a news conference. Television news channels reported as many as 15 people may have been injured by rocks thrown out by the volcano.

Japan has 110 active volcanoes and monitors 47 of them around the clock. In September 2014, 63 people were killed on Mount Ontake, the worst volcanic disaster in Japan for nearly 90 years.