The Meteorological Agency on Saturday issued an advisory regarding Mount Asama, following a rapid increase in seismic activity in the early morning. Smoke was seen billowing to a height of about 800 meters as of 6 a.m.
The active volcano straddles the border of Gunma and Nagano prefectures and is about 150 km northwest of Tokyo. The upmarket summer resort town of Karuizawa is on the mountain's southern slope.
The advisory was issued at 9 a.m. and by around 1 a.m., the number of volcanic quakes had begun to increase.