The local volcano observatory reported volcanic ash began spewing from Mount Sinabung early Sunday fell, along with small rocks, onto the villages of Sukameriah, Kutarayat and Kutagugung near the foot of the volcano, the Jakarta Globe reported Monday.
Five people were hospitalized with breathing difficulties, Antara reported.
Hundreds of villagers, although not under evacuation orders, left their homes after the eruption, emergency services officials said.
"The evacuees have been placed in buildings and houses around the Karo district office," Sutopo Purwo Nugroho, a spokesman for the National Disaster Mitigation Agency, said Sunday. "We're still gathering data on the number affected."
The alert level for Sinabung was raised to a level three on a four-point scale, with a level four alert meaning mandatory evacuation, the Globe said.
Armen Putra, an official at the Sinabung observatory, said after the initial eruption a series of volcanic earthquakes could be felt in six villages at the base of the mountain.