After a quiet spell, a series of volcanic earthquakes jolted the area around Mount Gamkonora, which started emitting towering columns of ash and smoke last week, said Saut Simatupang, head of Indonesia's Vulcanological Survey.
The flaming material spitting from the volcano, which towers above the island of Halmahera in North Maluku province, indicates magma is close to the crater's surface, officials say.
"After 18 hours of no activity, we have spotted five consecutive volcanic quakes this morning," Simatupang said by telephone from his office in the city of Bandung on Java island.
The alert status on the volcano remains at a maximum of four and officials fear the mountain is building up for a bigger eruption.
Clouds of smoke and ash had also grown thicker again, obscuring the volcano.
Simatupang said he received reports of 13 villages being covered in a thin shower of volcanic ash, forcing thousands more people to move into crowded makeshift shelters, or to stay with friends and relatives in neighbouring districts.
"Strong winds have carried clouds of volcanic ash in a radius of 15 kilometres (8 miles) from the crater's rim and it's heading northwards, posing a health hazard to many," he said.
Around 2,450 families or 12,500 people had been evacuated, many suffering respiratory problems, he said.
The 1,635 metre (5,400 foot) volcano is located about 2,400 km (1,491 miles) east of the capital Jakarta.
The last time ash and smoke streamed out of the volcano was in 1987. No casualties were reported.
Indonesia has the highest number of active volcanoes of any country, sitting on a belt of intense volcanic and seismic activity known as the "Pacific Ring of Fire".
- Reuters - Thomson Reuters Foundation
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