Indonesia has raised flight warnings around the Mount Sinabung volcano on Sumatra Island to their highest level after it sent a towering plume of ash more than 7 kilometres into the air, its biggest eruption this year.
Areas around the crater of the volcano, located about 1900km northwest of the capital Jakarta, have been off-limits for several years because of frequent volcanic activity.
The Australian Bureau of Meteorology's Volcanic Ash Advisory Centre in Darwin issued maps on Monday showing an ash cloud heading in three directions from Sinabung, to the north, northwest and south-southeast.
Indonesia also upgraded its Volcano Observatory Notice for Aviation to red, its highest warning, and said the ash-cloud top had reached 7,276 metres, according to a ground observer.
Sinabung is about 75km southwest of Kualanamu International Airport in Medan.
Nur Isnin Istianto, head of the regional airport authority, said Kutacane airport in Aceh province had been closed, but the wind direction allowed the airport of Kualanamu, Meulaboh and Silangit to remain open.
Sutopo Purwo Nugroho, a spokesman for Indonesia's Disaster Agency, said the eruption began on Monday morning, accompanied by multiple earthquakes and showering surrounding villages with small rocks.
"In five districts it became dark with a visibility of about 5 metres," he said in a statement.