TANAH KARO, Indonesia — An Indonesian volcano shot a towering cloud of black ash high into the air Tuesday, dusting villages 25 kilometres away in its most powerful eruption since awakening last week from four centuries of dormancy.

Some witnesses at the foot of Mount Sinabung reported seeing an orange glow -- presumably magma -- in cracks along the volcano's slopes for the first time.

"There was a huge, thunderous sound. It sounded like hundreds of bombs going off at one," said Ita Sitepu, 29, who was among thousands of people staying in crowded emergency shelters well away from the base. "Then everything starting shaking. I've never experienced anything like it."

Mount Sinabung's first eruption last week caught many scientists off guard. With more than 129 active volcanoes to watch in this vast archipelago, local vulcanologists had failed to monitor the long-quiet mountain for rising magma, slight uplifts in land and other signs of seismic activity.

Indonesia is a seismically charged region because of its location on the so-called "Ring of Fire" -- a series of fault lines stretching from the Western Hemisphere through Japan and Southeast Asia.

There are fears that current activity could foreshadow a much more destructive explosion in the coming weeks or months, though it is possible, too, that Singabung will go back to sleep after letting off steam.

More than 30,000 people living along the volcano's fertile slopes have been relocated to cramped refugee camps, mosques and churches in nearby villages.

But some have insisted on returning to the danger zone to check on their homes and their dust-covered crops.

The government sent dozens of trucks to the mountain to help carry them back before Tuesday's eruption, which sent ash and debris shooting 5,000 metres into the air, said Surono, who heads the nation's volcano alert center.

"It was really terrifying," said Anissa Siregar, 30, as she and her two children arrived at one of the makeshift camps, adding that the mountain shook violently for at least three minutes. "It just keeps getting worse."

Local media said ash had reached as far as Berastagi, a district 25 kilometres from the base of the mountain.

Surono, who, like many Indonesians, uses only one name, said activity was definitely on the rise: There were more than 80 volcanic earthquakes in the 24-hour lead-up to the blast, compared to 50 on Friday, when ash and debris shot nearly 3,000 metres.

The eruption early Tuesday occurred just after midnight during a torrential downpour. Witnesses said volcanic ash and mud oozed down the mountain's slopes, flooding into abandoned homes. Others said saw bursts of fire and hot ash.

The force of the explosion could be felt eight kilometres away.

Indonesia has recorded some of the largest eruptions in history.

The 1815 explosion of Mount Tambora buried the inhabitants of Sumbawa Island under searing ash, gas and rock, killing an estimated 88,000 people.

The 1883 eruption of Krakatoa could be heard 3,200 kilometres away and blackened skies region-wide for months. At least 36,000 people were killed in the blast and the tsunami that followed.