Indonesia issued a red alert Sunday after the Sinabung volcano on the island of Sumatra erupted, spewing smoke and ash 1,500 metres into the air and forcing the evacuation of thousands of people.
"Initially we thought the ash and smoke were triggered by rain but now we know the driving pressure was from magma," Surono, head of the nation's volcano disaster alert centre, told AFP.
"It's clearly dangerous so we've raised the warning to the highest level, or red level," he added.
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Sinabung, in northern Sumatra, has not erupted for more than 400 years but had shown "some volcanic activity" since Friday, Surono said.
"Our team is coordinating with district and provincial officials to monitor the situation," he added.
At least 12,000 people living nearby have been evacuated, the Antara news agency reported.
Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity.
