Mount Aso in Kumamoto Prefecture, southwestern Japan, has erupted.
The Meteorological Agency says the volcano erupted at 6:28 p.m. on Tuesday. Smoke mixed with volcanic ash rose about 200 meters from the mountain's crater.
The agency raised the alert level for Mount Aso to two on a scale of one to five on Sunday, after detecting active volcanic tremors and an increase in volcanic gas emissions.
People within roughly one kilometer of the crater are being urged to stay alert for huge airborne volcanic rocks and pyroclastic flows.
Mount Aso last erupted in October 2016.
Professor Takahiro Ohkura of Kyoto University is calling for close monitoring of Mount Aso.
He noted that Mount Aso has been rather active recently. He said Tuesday's eruption was very small, but that expansion of underground magma could lead to a bigger eruption. He said it is necessary to watch carefully how the volcano's activity proceeds.
