Indonesia on Thursday raised the danger alert level for a volcano that sparked a killer tsunami at the weekend, after previously warning that fresh activity at the crater threatened to trigger another deadly wave.

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Authorities also widened a no-go zone around Anak Krakatoa to five kilometres (three miles) -- up from a previous two kilometres -- and warned shell-shocked residents to stay away from the coast, after more than 400 were killed by Saturday night's killer wave.

Plumes of ash burst into the sky as pyroclastic flows -- hot gas and other volcanic material -- flowed down the crater, raising the risk of rough seas for boats in the vicinity.

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Authorities raised the crater's status to high alert, the second-highest danger warning, while aviation officials ordered flights to be redirected away from the area.

"We've raised the status of (the volcano) since this morning because there's been a change in the eruption pattern," Kus Hendratno, a senior official at the Krakatoa observatory, told AFP Thursday.

The new flows posed no immediate danger to area towns as the volcano sits in the middle of the Sunda Strait between Java and Sumatra islands.

But 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 okay."

A section of the crater -- which emerged at the site of the Krakatoa volcano, whose massive 1883 eruption killed at least 36,000 people -- collapsed after an eruption and slid into the ocean, triggering Saturday night's killer wave.

At least 430 people were killed, with 1,495 people injured and another 159 were missing.

Nearly 22,000 people have been evacuated and are living in shelters.

Indonesia, a vast Southeast Asian archipelago, is one of the most disaster-hit nations on Earth due to its position straddling the so-called Pacific Ring of Fire, where tectonic plates collide.

What caused the latest #Indonesia #tsunami with large loss of life? Potentially it was volcanic activity of Mount Anak Krakatau located in the middle of the Sunda Strait that divides Sumatra and Java. #SendaiFramework #ResilienceForAll https://t.co/cmi1rBXecC pic.twitter.com/IJ8q0Ep78e UNISDR (@unisdr) December 23, 2018

(AFP)

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