The Mount Merapi volcano on densely populated Java island is one of the most active in Indonesia and a series of eruptions in 2010 killed more than 350 people.
A disaster mitigation agency told residents living within a 5 km (3 mile) radius of the mountain to move to shelters, agency spokesman Sutopo Purwo Nugroho said in a statement.
He said that 120 people who had been hiking up Merapi were safe.
The airport in Yogyakarta, the nearest big city to the volcano, shut because of the threat from the ash, the state-owned aviation agency AirNav said in a statement.
The alert status on Merapi had not be raised, it said.
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