Prime Minister Yoshiro Mori pledged Wednesday that the government will spare no efforts in its monitoring of Mount Asama following the Meteorological Agency's issuance the previous night of an emergency warning of an impending eruption.
The 2,568-meter volcano straddles Gunma and Nagano prefectures.
"We will be on alert, sufficiently monitor and continue to pay attention to it," Mori told a group of reporters at his official residence.
Between 10 and 30 volcanic earthquakes per hour were occurring on the mountain Wednesday, the agency said.
The agency said in a bulletin at 8:50 p.m. Tuesday that the number of volcanic earthquakes on the mountain, which is about 140 km northwest of Tokyo, had soared to about 40 per hour, compared with less than 10 per day during a normal dormant period.
Mount Asama's seismic activity has risen to its highest level in the last two to three years, it said.
The mountain last erupted on a small scale on July 20, 1990.
The last eruption to claim casualties was in 1947, when 11 climbers died.