During Wednesday's spectacular eruption, the Ruang volcano, located on a remote island in North Sulawesi province, spewed a column of lava, red-hot rocks and ash three kilometers into the atmosphere.
Purple lightning lit up the sky above the erupting volcano, according to videos posted on social media.
"We're running, people," said a witness who filmed the eruption as he tried to evacuate. "We are running away because the ash is getting closer," he added.
More than 800 people were evacuated, and the authorities expanded the evacuation zone after the Volcanology Agency raised the alert level.
"The possibility of another eruption is still high, we must remain vigilant," Agency official Heruningtyas Desi Purnamasari told reporters on Thursday, blaming a rapid escalation of volcanic activity.
According to information received by the Agency, the rocks and ash damaged homes and forced the evacuation of a nearby hospital, the Indonesian official said.
Transport authorities have closed the airport in the provincial capital, Manado, as a protective measure against ashfall generated by the volcano.
Low-cost airline Air Asia has canceled flights from nine airports in East Malaysia and Brunei after authorities warned of a threat to passenger safety.
Officials have set up a six-kilometer perimeter around the volcano and are evacuating other residents, including some who live on the neighboring island of Tagulandang, said Abdul Muhari, a spokesman for the Disaster Management Agency.
About 1,500 people in high-risk areas had to be evacuated immediately, he added, and another 12,000 people could also be affected.
Indonesian officials have also signaled the risk of a tsunami if parts of the slope collapse into the ocean. About 400 people lost their lives in a tsunami caused by an earlier eruption of the Ruang volcano in 1871.
