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A 24-year-old man badly burned when Mount Sinabung spewed searing rocks and gas on Saturday died in hospital, while two other victims were being treated for serious injuries, said Sutopo Nugroho, spokesman for the National Disaster Management Agency.
Fourteen bodies were found hours after Saturday's eruption.
"There are reports that some more bodies have been found but we don't know how many," Nugroho said.
Mount Sinabung, which has been rumbling since September, erupted again on Saturday after a few days of lull when some residents began returning to their homes nearby.
The disaster agency said pyroclastic flows from the latest eruption traveled down the volcano's slopes, destroying everything on their path.
Earlier on Sunday, an aid worker said at least 50 people were still missing.
"We lost contact with 50 residents," said Benny Kaban, a local Protestant minister and aid worker. "They usually returned to their shelters at 6 in the evening."
Most of the 15 victims died in a village less near the peak of Mount Sinabung, officials said.
Local media reported that seven of those killed were students of a Christian university in Medan, the capital of North Sumatra province, who were volunteering to help displaced people.
More than 30,000 people have sought refuge in government buildings, churches, schools and mosques since the latest round of eruptions began four months ago.
President Susilo Bambang Yudhoyono on Sunday expressed condolences for the deaths and urged residents to be patient.