Indonesian authorities have raised the danger alert level for the Anak Krakatau volcano and ordered all flights to steer clear.
It comes days after it triggered a tsunami that killed more than 420 people.
The alert level for the erupting volcano was raised to the second highest and a three-mile exclusion zone imposed around the island, as residents were urged to stay away from the coast.
A crater collapse on the volcanic island at high tide on Saturday sent waves up to 16ft high smashing into the coast on the Sunda Strait, between the islands of Java and Bali.
The disaster struck without warning, taking people by surprise in a country that regularly suffers landslides, earthquakes and volcanic eruptions, and heavy rain and high seas have hampered the search and effort.
According to Indonesia's Disaster Mitigation Agency, more than 128 people are still missing.
Authorities have warned that the crater of Anak Krakatau remains fragile, raising fears of another collapse and tsunami.
People inspect the damage at a tsunami-ravaged village in Sumur, Indonesia | Image: Fauzy Chaniago/AP/Press Association Images
There are also fears of a bigger eruption and many residents are already refusing to return to their communities over fears of another tsunami.
The volcano has been rumbling on and off since July but has been particularly active since Sunday, spewing lava and rocks, and sending clouds of ash up to 3,000 metres into the sky.
A thin layer of volcanic ash has been settling on buildings, vehicles and vegetation along the west coast of Java since late on Wednesday, according to images shared by the national disaster mitigation agency.
Authorities said the ash was not dangerous but advised residents to wear masks and goggles when outside, while aircraft were ordered away.
"All flights are rerouted due to Krakatau volcano ash on red alert," the government air-traffic control agency AirNav said.
The civil aviation authority said no airports would be affected. The capital, Jakarta, is about 155 km east of the volcano.
