© ReutersVillagers fled as ash clouds spewed from the mountain
Mount Sinabung, which has been active with low-level eruptions for months, spewed clouds of hot ash into the air, engulfing nearby villages.
Emergency official Sutopo Purwo Nugroho said a group of schoolchildren on a sightseeing trip were among the dead.
The volcano began to rumble last September after being dormant for three years.
Thousands had been evacuated from their homes.
On Friday the disaster management agency had allowed people living in villages more than 5km (3 miles) from the mountain to return home.
But a day later, Sinabung spewed hot rocks and ash up to 2km in the air.
When the volcano erupted in 2010, at least two people were killed and 30,000 others were displaced.
Before 2010, Mount Sinabung had not erupted in 400 years. It is one of about 130 active volcanoes in Indonesia.
© APFor the past five months, Indonesians have become accustomed to eruptions from Sinabung
© APThe volcano has covered villages nearby in ash and debris
© APBut Saturday's eruption was the worst since Sinabung came back to life in 2010, after 400 years dormancy
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