Japan has raised a volcano warning level, urging residents near Sakurajima in the south-western region to prepare for evacuation.
The active volcano on the island of Kyushu is located about 50km from the Sendai nuclear plant, where the reactor was restarted on Tuesday despite warnings about geological instability.
The reactor was the first to come back online under new regulations imposed following the nuclear disaster at Fukushima four and a half years ago.
Japan is located on the Pacific "ring of fire", known for frequent earthquakes and volcanic eruptions.
In May, Mount Shindake on Kuchinoerabu Island near Sakurajima erupted, prompting authorities to evacuate the entire island.
