Airlines from Malaysia and Singapore have suspended flights to two cities near Indonesia's Mount Merapi, from which ash clouds and hot gas continue to spew.
The cities, Solo and Yogyakarta, are about 30km from the crater.
Scientists are warning the eruptions could continue for weeks.
Meanwhile, volcanic activity on Anak Krakatau, located on an island between Sumatra and Java, is also increasing, with as many as 700 instances of seismic movements recorded in a day.
Al Jazeera's Step Vaessen reports from Jakarta.