It was not immediately clear if the eruption was a direct result of the massive 8.9-magnitude earthquake that rocked northern areas on Friday, unleashing a fierce tsunami and sparking fears that more than 10,000 may have been killed.
The 1,421-metre (4,689-feet) Shinmoedake volcano in the Kirishima range saw its first major eruption for 52 years in January. There had not been any major activity at the site since March 1.
Authorities have maintained a volcano warning at a level of three out of five, restricting access to the entire mountain.
In April last year, the eruption of the Eyjafjoell volcano in Iceland dispersed a vast cloud of ash, triggering a huge shutdown of airspace that affected more than 100,000 flights and eight million passengers.
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