People are being urged to stay away from Mount Asama in central Japan following a minor eruption at the volcano on Wednesday night.
The eruption occurred shortly after 10 p.m. at the mountain, which straddles Nagano and Gunma prefectures, prompting the Meteorological Agency to raise the volcanic alert level from 1 to 3, on a scale of 5.
No new eruption has been observed since.
Agency officials said smoke from the crater reached more than 1,800 meters above the mountain and large rocks went flying as far as 200 meters from the crater.
They said volcanic ash has been observed in some areas of Gunma Prefecture.
The Agency has sent members to investigate the situation, including how far volcanic ash has travelled.
The eruption on Wednesday night came amid a fall in the number of volcanic quakes below the crater. About 10 such quakes per day have been observed this month, compared to around 50 quakes per day in July.
Agency officials say the number of volcanic quakes has been on the rise again since the eruption, with 25 observed on Thursday by 10 a.m. But officials say no crustal movement suggesting a new build-up of magma has been observed.
The agency is maintaining the alert level at 3, citing the possibility of a fresh eruption affecting places near residential areas.
It's calling on people within four kilometers of the crater to stay on the alert for possible large flying rocks, and pyroclastic flows of hot gas, ash and rocks.
It is also warning people outside the four-kilometer area of the possibility of winds carrying small volcanic rocks or windows being shattered by vibrations from an eruption.