Authorities warned on Friday that a volcano a few dozen kilometers from the Sendai nuclear plant in Kagoshima Prefecture was showing signs of increased activity and may erupt. It warned people to stay away from the summit.

The warning comes nearly a month after another volcano, Mount Ontake, erupted suddenly while it was crowded with hikers, killing at least 57 people in Japan's worst volcanic disaster in nearly 90 years.

Ioyama, a mountain on the southwestern island of Kyushu, has been shaken by small tremors and other signs of rising volcanic activity recently, including a tremor lasting as long as seven minutes, an official at the Meteorological Agency's volcano division said.