The victims died as the volcano sent lava and a searing cloud of super-heated gas cascading down its slopes.
Vulcanologists say Karangetang spewed in the middle of the night after heavy rain caused its hot lava dome to split.
Ash and lava then poured down the mountain’s western slope, according to the National Disaster Management Agency.
About 65 people were evacuated from threatened villages near the volcano’s flanks following the initial phase of the eruption.
The loss of one bridge due to the volcanic debris left about 20,000 residents of six villages cut off from the rest of their island.
The 5,800 foot-high volcano is located at the northern end of the Siau Island, north of the much larger island of Sulawesi.
It has produced more than 40 major eruptions since 1675. Indonesia has 128 active volcanos, 65 of which are listed as dangerous.
Photo: Center of Volcanology and Geological Hazard Mitigation