Mount Dukono, on Halmahera island in North Maluku province, erupted on Friday morning, spewing volcanic ash as high as 10km into the sky, the country's volcanology agency said.
The agency maintained the alert status at the mountain at its third-highest level, agency head Lana Saria said in a statement.
Video from the agency showed a huge cloud of hot ash billowing from the crater and blanketing the slopes of the volcano.
Volcanic activities in Mount Dukono decreased in 2025 but picked up again at the end of March, with nearly 200 small-scale eruptions.
The local rescue agency said earlier that 20 hikers were trapped by the eruption, including nine Singaporeans and 11 Indonesians.
Iwan Ramdani, the head of the local rescue agency, told Reuters that 17 had been found and evacuated.
Iwan said the search was still on for the three hikers who remained missing.
Survivors said three people, including two Singaporeans, died in the eruption, local police chief Erlichson Pasaribu told Reuters, but the rescue agency has yet to confirm the deaths.
Pasaribu said climbing in Mount Dukono had been banned since an earlier eruption in 2024.
Authorities warned residents to refrain from any activities within 4km of the crater.
There are not yet any reports of flight disruptions caused by the eruption.
Indonesia sits on the Pacific "Ring of Fire", an area of high seismic activity atop various tectonic plates.
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