About 15,000 people have been evacuated from 17 villages in the danger zone 5 kilometers (3 miles) around the crater, National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho said. The evacuation zone was expanded from 3 kilometers.
Thick, gray ash covered villages, farms and trees as far as 70 kilometers (43 miles) north of Mount Sinabung's crater, hitting the towns of Binjai and Langkat.
"Everything turned hot surrounding us," said Jatah Surbakti, a 45-year-old farmer who fled with his wife and four children to a shelter on trucks provided by the local disaster agency, along with hundreds other villagers.
"We were running in panic under the rain of ash and gravel. ... I heard many women and children screaming and crying," he said, adding that his fruit and vegetable farms were destroyed by the ash and his children's schools were disrupted.
The 2,600-meter (8,530-foot) Mount Sinabung has sporadically erupted since September. An eruption in 2010 killed two people and caught scientists off guard because the volcano had been quiet for four centuries.
Transportation Ministry spokesman Bambang Ervan said airlines had been notified to avoid routes near the mountain.
Mount Sinabung is among around 129 active volcanoes in Indonesia, which is prone to seismic upheaval due to its location on the Pacific "Ring of Fire," an arc of volcanoes and fault lines encircling the Pacific Basin.