Mount Sinabung spewed hot gas, ash and rocks 1.5 miles into the air in a series of eruptions during the morning.
Emergency official Sutopo Purwo Nugroho said three schoolchildren and a teacher were among the dead.
Thousands of people were evacuated in September when Sinabung erupted after being dormant for three years. Many were allowed back to their homes on Friday.
Officials fear there may have been more casualties, but they cannot get closer because of the heat from the eruption.
Pictures taken at the scene showed rescue workers recovering bodies that were buried in ash.
When Sinabung volcano erupted in 2010, at least two people were killed and 30,000 others were displaced. Before 2010, Sinabung had not erupted in 400 years.
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