"We must clear the island because we anticipate there could be more eruptions. No activity is allowed within four kilometres from the crater," she said.
Footage seen by Reuters showed flows of red lava streaming down the mountain, reflected in the waters below, and billowing clouds of grey ash above its crater.
Ruang island is home to about 838 residents, most of whom have now been evacuated to the nearest island Tagulandang, said Heruningtyas.
Indonesia straddles the so-called "Pacific Ring of Fire," an area of high seismic activity that rests atop multiple tectonic plates.