Experts say the long dormant period has made Mount Sinabung highly unpredictable [AFP]

A volcano on Indonesia's western-most island of Sumatra has unleashed its most violent eruption so far after laying dormant for 400 years.

Mount Sinabung sent a tower of hot ash more than three kilometres into the sky on Friday, sending thousands of residents fleeing to safety for the second time this week.

The tremor from the eruption in North Sumatra province could be felt some eight kilometres away.

Officials said it was the strongest eruption since the 2,460-metre volcano abruptly ended its long sleep on Sunday, and sparked panic among local residents.

"The volcano erupted at 4:38am and lasted for 13 minutes, sending a column of ash as high as 3,000 metres into the air. This is the biggest eruption," Agus Budianto, a government volcanologist, told the AFP news agency.

On alert

Budianto said there had been "intense magma movement" inside the volcano since Thursday evening.

"We must remain on alert for unpredictable events as this mountain has been dormant for hundreds of years," he added.

Mount Sinabung last erupted in the 1600s, and government scientists acknowledged they had made no efforts before the mountain started rumbling last week to sample gases or look out for rising magma or other signs of seismic activity.

Experts claimed they were too busy with more than 129 active volcanoes in Indonesia, 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.

The sudden initial eruption on Sunday and Monday forced 30,000 people living along the mountain's fertile slopes to evacuate to cramped emergency shelters in nearby towns.

Evacuation

Police have evacuated thousands of villagers from a six-kilometre danger zone around the mountain, but some residents are refusing to leave their homes on the slopes.

Some 30,000 residents were forced to flee their homes in the wee hours following the latest eruption [EPA]

Mount Sinabung is near Lake Toba, a 100-kilometre long volcanic crater that some archaeologists believe almost wiped out the human race when it erupted 69,000-77,000 years ago.

The Indonesian archipelago nation has recorded some of the largest eruptions in history.

Earlier this month four people went missing after the 1,784-metre Mount Karangetang erupted on the island of Siau, in North Sulawesi province.

The explosion of Mount Tambora in 1815 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.

The blast and the ensuing tsunami killed at least 36,000 people.

Source: Agencies