Bowo Asa, senior official at the disaster management and mitigation agency at North Sumatra, told Xinhua that about 12,000 people living in more than 10 villages have been evacuated to government camps since Sunday, while more than 6,000 others have already fled to different shelters.
The 2,475-metre-high Mount Sinabung belched a column of dark ash 1,500 metres high, leading the authorities to step up the alert status to the highest, Surono, head of national volcanology agency, said.
"Hot ash spreads 1,000 metres toward southeast of the crater," he said.
That led the agency to extend the evacuation zone from three to five km from the fiery crater, added Surono.
Indonesia is home to 129 active volcanoes and is located on a quake prone zone.
Indonesia has issued a warning to all international and domestic flights to re-route their pathway near Mount Sinabung since the volcano erupted Nov 18, the biggest eruption since the volcano first rumbled back to life in September after being dormant for three years. Mount Sinabung has erupted intermittently since then.
Transport Ministry Spokesman Bambang Ervan said the volcanic ash of the volcano was found up to 25,000 metres high in the air, endangering flights.
Indonesia, a vast archipelago country with 17,500 islands, is home to 129 active volcanoes and is located on a vulnerable earthquake-prone zone known as the 'Pacific Ring of Fire'.