Tangkuban Perahu, a smouldering 2,076-metre (6,933 foot) mountain near the city of Bandung on Java island, began rumbling overnight, prompting scientists to raise the status of the volcano to "alert" and declare it off limits.

A day earlier, more than 20,000 people fled the slopes of Mount Talang on Sumatra island, as the peak spewed hot ash after being unsettled by a series of huge quakes from the same faultline that caused last year's deadly tsunami.

There was new panic on Wednesday as a volcanic earthquake struck Talang at 10:00 am (0300 GMT), causing many to rush out of the buildings, mosques and schools that they have been sheltering in since evacuating their villages.

Indonesia has more than 130 active volcanoes and endures daily earthquakes attributed to the Pacific "ring of fire" -- the junction of three continental plates, which shift relentlessly with huge force under the country's islands.

But nerves have been sorely tested by two recent giant quakes, among the largest recorded in the past century, which have claimed thousands of lives and generated intense speculation over an impending third disaster.

On December 26 last year, a massive 9.3 magnitude earthquake triggered a tsunami that crashed onto Indian Ocean shorelines killing more than 220,000 people, most of them Indonesians.

Three months later the same faultine sent out a seismic shockwave measuring 8.7 on the Richter scale, causing extensive destruction on the Sumatra coast islands of Nias and Simeulue and leaving more than 670 people dead.

Thousands of people on the islands have refused to leave temporary hilltop camps, with forecasts by scientists of a third impending disaster fuelling rumours that a quake and tsunami could strike at any time.

A strong 6.7 aftershock that hit the Sumatra coast city of Padang late Sunday prompted a similar evacuation, leaving many markets, schools and office buildings deserted.

At Mount Talang, 40 kilometres (25 miles) east of Padang, scientists were closely monitoring the volcano'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 Tuesday, there had been several smaller explosions and ash emissions, but no signs of an impending major eruption.

Talang has had at least four major eruptions, all in the 19th century, and three smaller eruptions in 1981, 2001 and 2003.

At Java island's Tangkuban Perahu, a popular tourist spot that has an access road leading almost all the way to its crater, he said there had been almost continuous activity overnight.

"There have been hundreds of volcanic earthquakes recorded. And therefore this morning, at 8:25 am, the status of the volcano has been raised to 'alert'," he said.

Some 50 heads of state including China's President Hu Jintao and Japanese Prime Minister Junichiro Koizumi are due to visit the nearby city of Bandung this month to celebrate the golden anniversary of the Asia-Africa summit.