Disaster Mitigation Agency spokesperson Sutopo Purwo Nugroho said residents living within a five-kilometre radius of the mountain have been told to evacuate and move to shelters, Reuters reported. There were no immediate reports of casualties, and a group of 120 people who were hiking up Merapi are safe, officials said.

The airport in the nearest big city, Yogyakarta, was shut for about half an hour because of the ash.

Indonesia has nearly 130 active volcanoes, more than any other country. The country is prone to earthquakes and volcanic activity as it is on the Pacific Ring of Fire, a horseshoe-shaped series of fault lines. Reuters