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.

“There is an increase in activity that under certain circumstances could even lead to a small scale eruption, but it is not in danger of an imminent, major eruption,” the official said.

The warning level on the mountain has been raised from the lowest possible level, normal, to the second-lowest, which means that the area around the crater is dangerous, he added.

Ioyama lies in the volcanically active Kirishima mountain range and is roughly 64 km from the Sendai nuclear plant, which is run by Kyushu Electric Power Co. The Japanese government wants to restart the plant even though the public remains opposed to nuclear power following the Fukushima crisis.

Critics point out that the Sendai plant is only 50 km from Mount Sakurajima, a highly active volcano that erupts frequently. There are five giant calderas — crater-like depressions formed by past eruptions — in the region, and the closest one is 40 km away.

The plant still needs to pass operational safety checks as well as gain the approval of local authorities, which means it may not restart this year.

Before giving its initial green light to restart the plant in July, the Nuclear Regulation Authority said the chance of major volcanic activity during the life span of the Sendai nuclear plant was negligible.

On Friday, the warning level for the Sakurajima volcano was at 3, which means people should not approach the peak.

Japan lies on the “Ring of Fire” — a horseshoe-shaped band of fault lines and volcanoes around the edges of the Pacific Ocean — and is home to more than 100 active volcanoes.

Experts warn that the mammoth magnitude-9.0 earthquake in March 2011 may have increased the risk of volcanic activity throughout the nation, including that of iconic Mount Fuji.