TOKYO (Reuters) - Japan warned on Saturday that a volcano 50 km (31 miles) from a just-restarted nuclear reactor is showing signs of increased activity, and said nearby residents should prepare to evacuate.

Sakurajima, a mountain on the southern island of Kyushu, is one of Japan's most active volcanoes and erupts almost constantly. But a larger than usual eruption could be in the offing, an official at the Japan Meteorological Agency said.

"There is the danger that stones could rain down on areas near the mountain's base, so we are warning residents of those areas to be ready to evacuate if needed," the official added.

The agency also said it had raised the warning level on the peak, 990 km southwest of Tokyo, to an unprecedented 4, for prepare to evacuate, from 3. Roughly 100 people could be affected.

Japan on Tuesday restarted a reactor at the Sendai nuclear plant, some 50 km from Sakurajima. It is the first reactor to be restarted under new safety standards put in place after the 2011 Fukushima disaster.

Critics have long pointed out that the plant is also located near five giant calderas, crater-like depressions formed by past eruptions, with the closest one some 40 km away.

Still, the Nuclear Regulation Authority (NRA) has said the chance of major volcanic activity during the lifespan of the Sendai plant is negligible.

Two years ago, Sakurajima shot ash some 5,000 meters into the air.

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.

Last year, Mount Ontake in central Japan erupted unexpectedly, killing 63, the worst volcanic disaster for nearly 90 years. In May, a remote island south of Kyushu was evacuated due to another eruption.

(Reporting by Elaine Lies; Editing by Richard Borsuk)