Mount Ibu, located on remote Halmahera island in eastern Indonesia, erupted on Thursday morning, belching a tower of volcanic ash 5,000 metres (16,400 feet) above its peak.
The country's geology agency subsequently raised the status to the highest alert level of a four-tiered system, meaning an eruption is ongoing.
"Based on the visual and instrumental monitoring that shows the increasing volcanic activities of Mount Ibu, the alert level is raised from level three... to level four," agency head Muhammad Wafid said in a statement.
Ibu has been showing significantly increasing volcanic activity since mid-April and a series of earthquakes indicated mounting pressure because of magma migrating to the volcano's surface, Wafid said.
Residents living near Ibu and tourists have been advised to stay out of a four to seven-kilometre exclusion zone from the peak and to wear a face mask in case of falling ash.
Ibu is one of Indonesia's most active volcanos, erupting more than 21,000 times last year.
More than 700,000 people lived on Halmahera island as of 2022, according to official figures.
Indonesia, a vast archipelago nation, experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire".
Last month, Mount Ruang in North Sulawesi province erupted more than half a dozen times, forcing thousands of residents of nearby islands to evacuate.
Its status has since been lowered to the second-highest alert level.
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