Japan raised the volcano alert at Mt Hakone southwest of Tokyo, restricting access to the area after an apparent small eruption at the mountain popular with holiday-makers.
The Japan Meteorological Agency said it had confirmed volcanic ash was emitted from the area, which is known for hot springs and which has been emitting unusual amounts of steam in recent months.
"There appears to have been a very small-scale volcanic eruption," the agency said on its website, raising the alert level to 3 from 2 on a scale of 5.
The higher alert restricts people from entering the area.
Japan, one of the most seismically active areas on earth, has seen an upturn in earthquake and small volcanic eruptions, including one that required the evacuation of a small island in south Japan.