Indonesia has raised the alert level for the erupting Anak Krakatau volcano to the second-highest, and ordered all flights to steer clear, days after it triggered a tsunami that killed at least 430 people.
A crater collapse on the volcanic island at high tide on Saturday sent waves up to five metres high smashing into the coast on the Sunda Strait, between the islands of Java and Bali.
Authorities have warned that the crater of Anak Krakatau, or child of Krakatau, remains fragile, raising fears of another collapse and tsunami, and have urged residents to stay away from the coast. There are also fears of a bigger eruption.
The volcano has been rumbling on and off since July but has been particularly active since Sunday, spewing lava and rocks, and sending huge clouds of ash up to 3,000 metres into heavily overcast skies.
The national geological agency, in raising the alert level to the second-highest, set a 5km exclusion zone around the island. "Since December 23, activity has not stopped ... We anticipate a further escalation," said Antonius Ratdomopurbo, secretary of the geological agency.
Satellite photos aren't available because of cloud cover but radar images from a Japan Aerospace Exploration Agency satellite taken before and after the eruption show the volcano's southwestern flank has disappeared.
Dave Petley, head of research and innovation at Sheffield University who analysed similar images from a European Space Agency satellite, said they support the theory that a landslide, most of it undersea, caused the tsunami.
"The challenge now is to interpret what might be happening on the volcano, and what might happen next," he wrote in a blog.
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 in a release.
The civil aviation authority said no airports would be affected. The capital, Jakarta, is about 155km east of the volcano.
The status change sparked new fears, with many local residents already scared and refusing to return to their communities over fears of another tsunami.
"This worries me," said Ugi Sugiarti, a cook at the Augusta Hotel in hard-hit Carita. "I've already left."
Sukma, a security guard at the shattered Mutiara Carita Cottages, added: "Just please pray for us and that everything will be OK."
Nearly 22,000 people have been evacuated and are living in shelters.
In 1883, the volcano then known as Krakatoa erupted in one of the biggest blasts in recorded history, killing more than 36,000 people in a series of tsunami and lowering the global surface temperature by 1C with its ash.
Anak Krakatau is the island that emerged from the area in 1927 and has been growing ever since. Indonesia is a vast archipelago that sits on the Pacific "Ring of Fire".