Head of Indonesia's volcanology and geological hazard mitigation centre, Pak Kasbani, warned that lava could reach roads near the volcano. He also described how the volcano had spewed "quite intense" hot clouds at least 30 times since Wednesday morning.
There have been no immediate reports of damage, although photographs on social media depicted ash covering several houses.
The local disaster mitigation agency said that some residents had decided to leave their homes.
Indonesia is located on the “Ring of Fire” and, with nearly 130 active volcanoes, is home to more volcanoes than any other country.
Merapi is one of the country’s most active volcanoes, with its last major eruption in 2010 killing almost 350 people.
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