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 sending thousands of people fleeing from their homes.
A thick blanket of acrid black smoke is shrouding the area, disaster officials said, although no casualties have yet been reported.
"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.
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"It's clearly dangerous so we've raised the warning to the highest level, or red level," he added. "From the crater, it shot smoke and volcanic ash 1,500 metres (5,000 feet) into the sky."
The 2,460-metre (8,100 feet) 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.
Disaster Management Agency spokesman Priyadi Kardono told AFP that "thousands of residents" in four affected villages at the foot of the volcano had been evacuated.
He said there had been no reports yet of any deaths or injuries.
"Many had left their homes even before they were evacuated. They said the volcano was spewing thick black smoke, small stones and sulphur. They were so scared they decided to leave their homes and go to the city," Kardono said, adding that a rescue team has been sent to survey the area.
"The area is blanketed with thick smoke and there's a strong smell of sulphur. We foresee respiratory problems from the inhaling dust. Facemasks have been distributed," he added.
The Antara news agency said that at least 12,000 people living nearby had been evacuated.
Indonesia sits on the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity. It has more active volcanoes than any other country.
Earlier this month, four people went missing after the 1,784-metre (5,853-foot) Mount Karangetang, on the remote island of Siau in North Sulawesi province, erupted.
Mount Baru Jari on Lombok island, near the resort island of Bali, erupted in May, spewing ash and lava at least 1,500 metres into the sky, damaging crops but not threatening villagers.
