A second Indonesian volcano has sprung to life after a series of earthquakes, intensifying fears that the archipelago's violent geological forces will unleash a new disaster.
Tangkuban Perahu, a smouldering 2,076-metre mountain near the city of Bandung on Java island, has started rumbling.
Scientists raised the alarm and declared the summit around the open crater off limits.
"There is possibility that poisonous gas may come out," said Surono, a vulcanology and geological disaster mitigation official.
Surono said the volcano's status had been raised from "alert" to "prepare".
More than 20,000 people had already fled the slopes of Mount Talang on Sumatra island, as its peak spewed hot ash.
President Susilo Bambang Yudhoyono, who has called an emergency summit of regional governors to discuss volcanoes, was due to visit Talang to try to calm some of people who had abandoned their homes.
Scientists were closely monitoring Mount Talang's activity after having raised its status to "beware", one rung short of a full-blown eruption.
Syamsurizal, a geologist at Indonesia's vulcanology headquarters in Bandung, said that since an outburst early on Tuesday, there had been several smaller explosions and ash emissions, but no signs of an impending major eruption.
Mas Ace Purbawinata, a senior geologist deployed to Talang, told ElShinta radio that the volcano appeared to be calming down.
But he said the tremors indicated that molten lava was trying to force its way through the earth's crust.
"The [frequency of] volcanic tremors is still quite high," he said.
Talang has had at least four major eruptions, all in the 19th century, and three smaller eruptions in 1981, 2001 and 2003.
Indonesia has more than 130 active volcanoes and endures daily seismic jolts attributed to the Pacific 'ring of fire' - restless fissures in the earth's crust which cause seismic activity from Japan to the Indian Ocean.