Airlines have been advised to stay clear of the erupting Mount Sinabung volcano in North Sumatra.
Indonesian officials have raised the status of the volcanic activity to the highest level.
A number of domestic and regional flights have been disrupted.
Mt Sinabung has been showing signs of life since September, after being dormant for three years.
Over the weekend, it shot ash and rocks far into the air, prompting the alert move.
The ash reached the provincial capital of Medan, some 80 kilometres away.
So far, more than 12,000 people have been evacuated from 19 villages affected by volcanic ash.
