Mount Kelud in East Java, Indonesia, started eruption at 22:50 p.m. Thursday, forcing thousands of residence living within 10 km from the crater to evacuate, a scientist at the national volcanology agency said.
"The rains of gravel and sand have reached 15 km from the crater, hitting and impairing those being evacuated. This means that the evacuees must go farther than 15 km," Xinhua quoted Umar Rosadi, a volcanologist, as saying.
Based on data recorded days before the eruption, the official said that there are at least 21,000 people living in the dangerous zone.
The volcanology agency elevated the alert status of Mount Kelud to the top level at 21:15, according to Rosadi.
The last volcanic eruption of Mount Kelud occurred in 2007 and its strongest eruption took place in 1919 and claimed 55 lives.
Indonesia has just endured months long volcanic eruption of Mount Sinabung in North Sumatra that killed 17 people.
Mount Kelud is among 129 active volcanoes in Indonesia, which is prone to seismic upheaval as it lies on the Pacific "Ring of Fire", an arc of volcano and fault lines surrounding the Pacific basin.