Central Japan's Mount Asama volcano is in danger of erupting, the country's Meteorological Agency warned on Sunday after detecting an increase in seismic activity.
The agency raised the alert level, urging residents nearby to take caution.
"Mount Asama could erupt within two days, spewing rocks and ash. We are carefully monitoring its seismic activity," said agency official Sadayuki Kitagawa.
Located about 145 km northwest of Tokyo, Asama is one of the nation's most active volcanoes. Its last major eruption took place in September 2004, the agency said.
An eruption of the 8,425-foot-high (2,568-meter-high) volcano could shoot rocks and ash in a 4 km radius. While no one lives in that danger zone, some 45,000 other nearby residents may have to evacuate as a precaution, police said.
Today's alert level was raised to three, urging awareness and caution. Alert level four advises residents to prepare for evacuation, while level five, the highest, orders evacuation, according to the agency.
Jou Hagiwara, an official of Naganohara town near the volcano, said authorities are calling on its 6,800 residents via speakers to be alert for possible eruptions.
"We urge people not to go near the volcano, and keep residents updated with the latest seismic information," Hagiwara said.
With 108 active volcanos, Japan is among the most seismically busy countries in the world. The country lies in the "Ring of Fire" -- a series of volcanoes and fault lines that outline the Pacific Ocean.