The country's volcanology agency had warned the threat from the volcano was not over after it erupted more than half a dozen times this month, sparking the evacuation of more than 6,000 people.
Ruang, located in Indonesia's North Sulawesi province, erupted at around 01:15 am local time on Tuesday (1715 GMT Monday) and then twice more that morning, the volcanology agency said in a statement.
The volcano sent a tower of ash more than five kilometres (3.1 miles) into the sky, it added, as well as a fiery column of lava.
The national disaster agency BNPB estimated 11,000 to 12,000 people had to be relocated from near Ruang's crater, spokesman Abdul Muhari told a press conference.
"Currently local disaster mitigation agency... military and police are evacuating residents," he said.
Images released by the agency showed a molten red column bursting into the sky, a large ash cloud spilling from the crater and burning embers near local houses.
The disaster agency imposed a seven-kilometre exclusion zone around Ruang after volcanology officials warned locals of "the potential for ejections of incandescent rocks, hot clouds and tsunamis due to eruption material entering the sea".
Abdul said a rescue ship and a warship had been dispatched to help move thousands from neighbouring Tagulandang island north to Siau island because of the tsunami warning.
"We urge people in Tagulandang island to stay away from coastal areas, to be on alert for the potential of a tsunami to occur," he said, citing an 1871 wave that killed around 400 people after volcanic debris fell into the sea.
- Relocations -
The tsunami fears were also informed by more recent events.
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.
More than 800 people live on Ruang, all of whom were evacuated this month.
Some had returned to their homes after the emergency response ended on Monday, an AFP journalist said.
It was unclear how many residents had gone back and how many were forced to evacuate once more.
But Abdul said those who were previously evacuated from Ruang had been to be taken to provincial capital Manado to await relocation, unable to return to their homes because of the fear of eruptions.
Ruang's latest eruption prompted authorities to again close Sam Ratulangi international airport in Manado, more than 100 kilometres away, according to a notice from state-run air traffic control provider AirNav Indonesia.
The notice said the airport was shutting down due to "Ruang volcanic ash".
Indonesia, a vast archipelago nation, experiences frequent seismic and volcanic activity due to its position on the Pacific "Ring of Fire".
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