The Meteorological Agency warned Wednesday afternoon that Mount Bandai in Fukushima Prefecture could soon have a minor eruption.
It is the first time the agency has issued a volcanic advisory for the mountain since it started monitoring the 1,819-meter peak in 1965, an agency official said.
Volcanic advisories are issued when unusual volcanic phenomena are observed and when preparation for disaster prevention is considered necessary.
Tremors have been observed at the mountain since late April and their frequency has risen sharply since Monday, the official said. Over 170 tremors were recorded Monday, with the figure surging to 416 Tuesday -- the highest recorded by the agency. Of Tuesday's tremors, four were of magnitude 2 or higher, it said.
While the frequency of the quakes subsided somewhat Wednesday, the local observatory advised caution, saying that volcanic activity was still rising.
It also said it found that vapor had erupted Monday from a crater wall on the northern part of mountain top, rising as high as 100 meters, or five times normal.