Indonesia has nearly 130 active volcanoes, more than any other country, and while many show high levels of activity it can be weeks or even months before an eruption.
Raditya Jati, a spokesman for the agency, said in a statement that the eruption from the Mt. Ile Lewotolok volcano had caused panic among those living nearby.
Muhammad Ilham, a 17-year-old who witnessed the eruption, told Reuters that resident nearby were "panicked and they're still looking for refuge and in need of money right now".
Indonesia's Volcanology and Geological Hazard Mitigation Centre said on its website that the area near the volcano is likely to be inundated with "hot clouds, lava stream, lava avalanche, and poisonous gas".