Mount Hakone in Japan's Kanagawa prefecture erupted on Tuesday, the authorities said.
Earlier in the day, the Japan Meteorological Agency (JMA) raised its alert level from two to three on its scale of five, warning people to stay away from the volcano and its crater, Xinhua news agency reported.
It also said a larger and deadlier eruption could take place according to the latest seismic and geological activity detected and the residential region of Owakudani could be affected.
An evacuation advisory has been issued in the area within a one-km radius of the region.
The JMA has confirmed that fresh layers of volcanic ash had been detected near a newly-found vent in the Owakudani valley, after the volcano had been emitting irregular amounts of steam in recent months.
The agency sent a team of experts to the mountain on Monday morning, while Prime Minister Shinzo Abe's office set up an emergency liaison office to deal with the disaster and its potential escalation.
More From This Section
The JMA said it had detected 14 earthquakes on Monday, with magnitudes ranging between 1.9 and 3.2.
The higher alert restricts people from entering the area.
Mount Hakone last erupted 2,900 years ago.
--Indo-Asian News Service
ksk/pku/vm