Indonesia has raised the alert level for two volcanoes in the Sunda Strait and Mollucas island chain to the second highest following increased volcanic activities, a volcanology official said.
The alert status for Anak Krakatau, a volcano formed in the Sunda Strait after the legendary explosion of Mount Krakatau in 1883, and Ibu volcano on Halmahera island in the Mollucas was raised on Monday after rumbling to life about a week ago.
"Anak Krakatau's explosions are strong, so we urge people not to get close to it as it is throwing out flaming rocks," Surono, head of the volcanology centre in Bandung city, said.
People have been told to stay at least 2km from both volcanoes. Anak Krakatau, which means "Child of Krakatau", is a magnet for tourists while many inhabitants farm on the slopes of Ibu (Mother) volcano.
Scientists monitoring Anak Krakatau said the volcano was not especially dangerous and was likely to continue to rumble for some time.
Surono said the alert for Anak Krakatau will not be raised to the highest because nobody lives close to the mountain, which lies near Java island.
Anak Krakatau roared to life last November when it blasted massive clouds of smoke and flaming red rocks hundreds of metres into the sky.
The head of the country's volcanology centre said ashes from Ibu volcano had reached as high as 700 metres, but had not disturbed flights.
Indonesia has the highest number of active volcanoes of any country, sitting on a belt of intense seismic activity known as the "Pacific Ring of Fire".
People often live and farm on the slopes of volcanoes because of the rich volcanic soil.
In the past two years, at least two other volcanoes in Indonesia - Mount Merapi and Kelud - have shown signs of activity, but they are quiet now.