The volcano is about 100 miles northwest of Tokyo in Gunma Prefecture. Photos showed ash covering a large area of the mountainside below the three large craters at its peak. After the eruption about 10 a.m. Tuesday, officials raised the alert level, restricting access to the area.

“Seeing cinders falling in the videos, it is certain that an explosive eruption occurred,” Toshitsugu Fujii, a volcanologist and professor emeritus of the University of Tokyo, told NHK. He said that the volcano had erupted 3,000 years ago but that no activity had been detected recently.

“We need to be warned, as there may be another eruption,” he said.

Separately, the Mayon volcano in the Philippines continued to billow ash on Tuesday, one day after officials raised the alert level to four, the second-highest level. That means the volcano is in a period of “intense unrest” and could erupt hazardously at any time.

The Mayon volcano shot fountains of lava into the air early Monday, and the ash plume has climbed more than three miles high. The volcano, on the far southeastern corner of Luzon, the largest island of the Philippines, has had several deadly eruptions over the past two centuries. In 2006 more than 1,200 people were killed when a typhoon set off deadly flows of mud and ash that had been released by a recent eruption.