Mount Ruang erupted three times on Tuesday, spewing lava and ash more than five kilometres (three miles) into the sky and forcing authorities to issue evacuation orders for 12,000 locals.
A rescue ship and a warship were dispatched to help move thousands from neighbouring Tagulandang island north to Siau island because of a warning about parts of the volcano falling into the sea potentially causing a tsunami.
Rosalin Salindeho, a 95-year-old resident of Tagulandang in Indonesia's outermost region of North Sulawesi province, spoke of her fears when Ruang erupted after arriving in Siau.
"The mountain exploded. Wow, it was horrible. There were rains of rocks. Twice. The second one was really heavy, even the houses far away were also hit," she said.
The country's meteorological agency (BMKG) shared a map on Wednesday morning that showed volcanic ash had reached as far as eastern Malaysia on Borneo island, which the country shares with Indonesia and Brunei.
The spread of volcanic ash forced seven airports to close, the biggest in provincial capital Manado and the city of Gorontalo, according to a notice from state-run air traffic control provider AirNav Indonesia.
Julius Ramopolii, head of Mount Ruang monitoring post, said the volcano was still billowing ash and smoke above the crater on Wednesday morning.
"The volcano is visibly seen, the plume of smoke is visible, grey and thick, and reached 500-700 metres (2,300 feet) above the crater," he said in a statement.
He said the alert level remained at its highest of a four-tiered system and called on locals to remain outside of a seven-kilometre exclusion zone.
The tsunami fears were informed by recent experience.
The crater of Mount Anak Krakatoa, between Java and Sumatra islands, also partly collapsed in 2018 when a major eruption sent huge chunks of the volcano sliding into the ocean, triggering a tsunami that killed more than 400 people and injured thousands.
Indonesia, a vast archipelago nation, experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire".
