Eleven volcanoes are under close watch in Indonesia after a series of powerful quakes awoke intense subterranean forces and increased the chances of a major eruption, scientists have said.
As tens of thousands of people spent a third night in temporary camps after fleeing the slopes of Mount Talang on Sumatra island, where hot ash has been raining down since Monday, more volcanoes began rumbling.
Late on Wednesday, Anak Krakatau - the "child" of the legendary Krakatoa that blew itself apart in 1883 in one of the worst natural disasters recorded - was put on alert status amid warnings of poisonous gas emissions.
No one lives on Krakatau, which forms a small island in the Sunda Strait between Java and Sumatra, but the peak is a popular tourist spot, attracting Indonesian and foreign day-trippers.
Leaders to meet
A similar warning was earlier issued on Tangkuban Perahu, near the west Java island city of Bandung, which will next week host more than 50 Asian and African heads of state at a summit.
Isya Nur Ahmad Dana of Indonesia's Volcanology Office said on Thursday that Mount Merapi, 70km north of the Sumatran city of Padang, had been on alert since August, but along with seven other peaks was now under closer watch.
"The status of the Tangkuban Perahu in west Java and Krakatau in the Sunda Strait have both been raised from 'normal' to 'alert'
Isya Nur Ahmad Dana,
Indonesia's Volcanology Office
"The status of the Tangkuban Perahu in west Java and Krakatau in the Sunda Strait have both been raised from 'normal' to 'alert' on Wednesday following an observed increase in volcanic activities," Dana said.
Amid growing fears of an imminent disaster in the wake of recent powerful earthquakes and December's devastating tsunami, Indonesia's government has urged people to remain calm.
President Susilo Bambang Yudhoyono travelled to the area around Talang, 25km east of Padang, to meet some of the more than 20,000 people who have evacuated villages on the fertile slopes of the smoking peak.
Susilo's deputy Jusuf Kalla warned people living in the vicinity of other active volcanoes to take precautions and urged local officials to make contingency plans in anticipation of an eruption.
"We call on the people to really be alert," he said. Indonesia has more than 130 volcanoes, forming part of the Pacific Ring of Fire, an arc of intense seismic activity that stretches from quake-prone Japan to Southeast Asia and across the Pacific basin.
The archipelago nation's proximity to the junction of three continental plates, which collide with immense pressure, makes it vulnerable to earthquakes and eruptions.
Indonesia was hit by the tsunami
on 26 December
A massive magnitude 9 earthquake on 26 December triggered the Indian Ocean tsunami that killed more than 220,000 people. A second quake of 8.7 on the Richter scale from the same faultline killed 670 people last month.
Scientists have warned of a possible third disaster, with some predicting another quake and others an eruption from a so-called super volcano, such as the giant crater of Lake Toba on Sumatra island, where increased activity has also been recorded.
Scientists were unable to determine whether the peak of Mount Talang, a 2599m volcano that last erupted in 2003, was beginning to calm.
"Our team are still studying the data on site and we cannot yet say whether the activities of Mount Talang has slowed down or energy is building up for a bigger eruption," Dana said.
Krakatau and Tangkuban Perahu remained off limits to the public, with officials reportedly turning back dozens of sightseers.
"We have advised the concerned local government to close them for visits because of the possibilities of dangerous gas emissions or outbursts," said Dana.
He said there were no immediate efforts to evacuate the population around the Tangkuban Perahu, which straddles the territories of two districts and the city of Bandung, with a total population of 7.5 million people.