Thousands of Indonesians spent a third day in shelters Tuesday as a volcano threatened a devastating eruption on Sumatra island, officials said.

Governent volcanologists said 2,460-metre (8,000-foot) Mount Sinabung was not spewing as much ash and smoke into the atmosphere as it had for the first two days but it was still extremely dangerous.

The mountain erupted for the first time in 400 years on Sunday, sending around 30,000 people into temporary camps as authorities ordered everyone within a six-kilometre (four-mile) radius to leave.

Officials said about 7,000 people had returned to their villages Tuesday but those from within the evacuation zone were not being allowed to go home.

"People are asking us if they can go home to tend to their animals and guard their homes but they're not allowed to leave the camps," disaster response official Mohammad Agus Widisono said.

Government volcanologist Agus Budianto said: "The volcano continues to spew smoke and ash but the cloud is only 50 metres high compared to 2,000 metres yesterday. Visually, its activities have significantly reduced.

"But our instruments have recorded continuous tremors in the volcano, which means that there is magma trying to push upward."

He said the situation remained too precarious for people who live closest to the volcano to go home.

"We predict that there's still a possibility that it will erupt again," he said, adding there was "no plan" to downgrade the threat level.

Airlines have been warned to avoid Mount Sinabung in northern Sumatra but the area is remote and the ash cloud has caused minimal flight problems.

Sinabung is one of 69 active volcanoes in the sprawling Southeast Asian archipelago.

Indonesia straddles the Pacific "Ring of Fire", where the meeting of continental plates causes high volcanic and seismic activity. It has more active volcanoes than any other country.

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