The alert level was raised due to signs of increased seismic activity on Mount Asama, a 2,568 metre (8,425 foot) peak 140 km (90 miles) northwest of Tokyo, the official said.
"There are prospects for an eruption that could throw volcanic rocks to a distance of around 4 kilometres," Sadayuki Kitagawa, senior coordinator for volcanic affairs at Japan's Meteorological Agency, told Reuters by telephone.
He said the type of seismic activity detected on the mountain pointed to the possibility of an eruption within a few hours to around two days.
Mount Asama, one of Japan's more active volcanoes, had its biggest eruption in 21 years on Sept. 1 2004, spewing hot rock and raining ash on areas as far as 200 km (125 miles) away.
That eruption, however, did not cause any major damage. The warning level for a Mount Asama eruption was raised one notch to level three on a scale of five, in which five is the highest level of alert, Kitagawa said.
A level three alert covers non-residential areas near the crater of the volcano and warns people not to approach the volcano.
It was the first such alert for Mount Asama since the Meteorological Agency adopted the current volcanic warning levels in December 2007, Kitagawa said.
Mount Asama is known for a huge eruption in 1783 that caused widespread damage and killed around 1,500 people.
ReutersLast Mod: 01 Şubat 2009, 17:22