Almost 18,000 people have now fled their homes as a volcano violently erupts in western Indonesia, an official said, as it spewed more red-hot gas and rocks Monday.

Authorities at the weekend issued the highest safety alert for Mount Sinabung on the island of Sumatra after it erupted spectacularly, urging everyone in a five-kilometre (3.1-mile) radius of the volcano to evacuate.

On Monday, Sinabung erupted six times more, spewing columns of ash as high as 2,000 metres (6,500 feet), national disaster agency spokesman Sutopo Purwo Nugroho said.

"The number of displaced increased a lot because we lifted the alert level," he said, adding 17,713 people had now fled their homes up from around 12,000 the day before.

"But some communities are not as affected and people are staying at home to farm and rear livestock."

Sinabung is one of dozens of active volcanoes in Indonesia, which straddles major tectonic fault lines known as the "Ring of Fire".

In August, five people were killed and hundreds evacuated when a volcano on a tiny island in East Nusa Tenggara province erupted.

Mount Merapi in central Java, the country's most active volcano, killed more than 350 people in a series of violent eruptions in 2010 and showed signs of low activity last week.