Indonesian authorities have raised an alert for the erupting Anak Krakatau volcano to the second-highest level and ordered all flights to steer clear, days after it triggered a tsunami that killed at least 430 people.

The country’s geological agency has imposed a three-mile exclusion zone around the volcanic island and urged people to stay away from the coast amid fears a crater collapse could trigger further deadly waves.

A crater collapse at high tide on Saturday sent waves up to six metres high (20ft) smashing into the coast on the Sunda Strait, between the islands of Java and Sumatra.

The tsunami struck without warning, taking people by surprise in a nation that regularly suffers earthquakes, volcanic eruptions and landslides.

No earthquake shook the ground beforehand and the waves surged inland at night on a holiday weekend as people enjoyed beach concerts and other coastal activities.

Nearly 22,000 people have been displaced by the disaster, with about 1,500 injured and 159 still missing.

Authorities have warned that the crater of Anak Krakatau, or “child of Krakatoa”, remains fragile. 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.

“Since 23 December, activity has not stopped,” said Antonius Ratdomopurbo, secretary of the geological agency. “We anticipate a further escalation.”

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 Indonesia’s national disaster mitigation agency.

Authorities said the ash was not dangerous but advised people to wear masks and goggles when outside, and flights have been rerouted.

The civil aviation authority said no airports would be affected. Indonesia’s capital, Jakarta, is about 96 miles east of Anak Krakatau.

Indonesia tsunami:Rescue operation begins after devastating tidal wave Show all 25 1 /25 Indonesia tsunami:Rescue operation begins after devastating tidal wave Indonesia tsunami:Rescue operation begins after devastating tidal wave Debris littered a property badly damaged by a tsunami in Carita, Indonesia. The tsunami occurred after the eruption of a volcano around Indonesia's Sunda Strait during a busy holiday weekend, sending water crashing ashore and sweeping away hotels, hundreds of houses and people attending a beach concert AP Indonesia tsunami:Rescue operation begins after devastating tidal wave Damaged cars are seen among collapsed houses after a tsunami hit Banten, Indonesia, December 23, 2018 in this photo taken by Antara Foto. Antara Foto/Dian Triyuli Handoko/ via REUTERS ATTENTION EDITORS - THIS IMAGE WAS PROVIDED BY A THIRD PARTY. MANDATORY CREDIT. INDONESIA OUT. ANTARA FOTO Reuters Indonesia tsunami:Rescue operation begins after devastating tidal wave An aerial photo shows damaged buildings in Carita AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave Relatives mourn at a health facility where the bodies of tsunami victims are collected EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave A resident walks past a collapsed guesthouse after a tsunami hit the area at Carita beach in Pandeglang, Banten province, Indonesia, December 23, 2018. REUTERS/Stringer NO RESALES. NO ARCHIVES. STRINGER Reuters Indonesia tsunami:Rescue operation begins after devastating tidal wave epa07246244 Indonesian officers carry an injured person into an ambulance after a tsunami hit the Sunda Strait in Banten, Indonesia 23 December 2018. According to the Indonesian National Board for Disaster Management (BNPB), at least 43 people dead and 584 others have been injured after a tsunami hit the coastal regions of the Sunda Strait. EPA/TUBAGUS TUBAGUS EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave An aerial photo shows damaged buildings in Carita on December 23, 2018, after the area was hit by a tsunami on December 22 following an eruption of the Anak Krakatoa volcano. - A volcano-triggered tsunami has left at least 168 people dead and hundreds more injured after slamming without warning into beaches around Indonesia's Sunda Strait, officials said on December 23, 2018 voicing fears that the toll was set to rise. (Photo by Azwar Ipank / AFP)AZWAR IPANK/AFP/Getty Images AZWAR IPANK AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave epa07246251 A man walks among debris, past a damaged car, after a tsunami hit the Sunda Strait in Pandeglang, Banten, Indonesia, 23 December 2018. According to the Indonesian National Board for Disaster Management (BNPB), at least 43 people dead and 584 others have been injured after a tsunami hit the coastal regions of the Sunda Strait. EPA/ADI WEDA ADI WEDA EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave A man reacts after identifying his relative among the bodies of tsunami victims in Carita, Indonesia, Sunday, Dec. 23, 2018. The tsunami occurred after the eruption of a volcano around Indonesia's Sunda Strait during a busy holiday weekend, sending water crashing ashore and sweeping away hotels, hundreds of houses and people attending a beach concert. (AP Photo/Fauzy Chaniago) Fauzy Chaniago AP Indonesia tsunami:Rescue operation begins after devastating tidal wave TOPSHOT - An aerial photo shows damaged buildings in Carita on December 23, 2018, after the area was hit by a tsunami on December 22 following an eruption of the Anak Krakatoa volcano. - A volcano-triggered tsunami has left at least 168 people dead and hundreds more injured after slamming without warning into beaches around Indonesia's Sunda Strait, officials said on December 23, 2018 voicing fears that the toll was set to rise. (Photo by Azwar Ipank / AFP)AZWAR IPANK/AFP/Getty Images AZWAR IPANK AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave epa07246239 Dead bodies collected at a health facility after a tsunami hit Sunda Strait in Pandeglang, Banten, Indonesia, 23 December 2018. According to the Indonesian National Board for Disaster Management (BNPB), at least 43 people dead and 584 others have been injured after a tsunami hit the coastal regions of the Sunda Strait. EPA/ADI WEDA ADI WEDA EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave Residents collect debris from their collapsed houses Reuters Indonesia tsunami:Rescue operation begins after devastating tidal wave A local health center officer covers a dead body Reuters Indonesia tsunami:Rescue operation begins after devastating tidal wave This handout from Indonesia's Social Affairs Ministry taken and released on December 23, 2018 shows damaged buildings and vehicles in Carita, after the area was hit by a tsunami on December 22 following an eruption of the Anak Krakatoa volcano. - A tsunami following a volcanic eruption killed 62 people and injured hundreds more as it slammed without warning into tourist beaches and coastal areas around Indonesia's Sunda Strait on the night of December 22, sending panicked holidaymakers and residents fleeing. (Photo by Handout / Indonesia Social Affairs Ministry / AFP) / -----EDITORS NOTE --- RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / Indonesia Social Affairs Ministry" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTSHANDOUT/AFP/Getty Images HANDOUT AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave A man inspects his house which was damaged by a tsunami, in Carita, Indonesia, Sunday, Dec. 23, 2018. The tsunami apparently caused by the eruption of an island volcano killed a number of people around Indonesia's Sunda Strait, sending a wall of water crashing some 65 feet (20 meters) inland and sweeping away hundreds of houses including hotels, the government and witnesses said. (AP Photo/Dian Triyuli Handoko) Dian Triyuli Handoko AP Indonesia tsunami:Rescue operation begins after devastating tidal wave Bodies of victims recovered along Carita beach are placed in body bags on December 23, 2018, after the area was hit by a tsunami on December 22 following an eruption of the Anak Krakatoa volcano. - A tsunami following a volcanic eruption killed 62 people and injured hundreds more as it slammed without warning into tourist beaches and coastal areas around Indonesia's Sunda Strait on the night of December 22, sending panicked holidaymakers and residents fleeing. (Photo by Semi / AFP)SEMI/AFP/Getty Images SEMI AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave An Indonesian man looks at ruined vehicles EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave epa07246112 A handout photo made available by the Indonesia's national disaster management (BNPB) shows a ruined car that was rolled over after a tsunami hit Sunda Strait, in Anyer, Banten, Indonesia, 23 December 2018. According to BNPB, at least 43 people dead and 584 others have been injured. EPA/BNPB / HANDOUT HANDOUT EDITORIAL USE ONLY/NO SALES BNPB / HANDOUT EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave Indonesian men walk among debris EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave Rescue team evacuate the body of a victim EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave A ruined tractor sits in the water EPA Indonesia tsunami:Rescue operation begins after devastating tidal wave Survivors receive treatment at a hospital in Carita AFP/Getty Indonesia tsunami:Rescue operation begins after devastating tidal wave A resident walks past damaged houses AP Indonesia tsunami:Rescue operation begins after devastating tidal wave A soldier examines a local resident who was injured Antara Foto/Reuters Indonesia tsunami:Rescue operation begins after devastating tidal wave Damaged cars are seen among the debris after a tsunami hit the Sunda Strait in Indonesia EPA

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 tsunamis and lowering the global surface temperature by 1C with its ash.

Anak Krakatau, the island that emerged from the resulting hollow in 1927, has been growing ever since.

Indonesia is a vast archipelago that sits on the Pacific “Ring of Fire”, the site of many earthquakes and volcanic eruptions. This year, the country has suffered its worst annual death toll from disasters in more than a decade.

The latest has evoked memories of the 2004 Indian Ocean tsunami, triggered by an earthquake on Boxing Day, which killed 226,000 people in 14 countries, including more than 120,000 in Indonesia.

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Warning systems were subsequently set up but they have failed to prevent further disasters, often because apparatus has not been maintained properly, while public education and tsunami preparation efforts have been patchy.

Ramdi Tualfredi, a teacher in the village of Cigondong, on Java’s west coast, said efforts to prepare communities for tsunami had “totally failed”.

He said: “There were no preparations. I didn’t get information from anywhere.” He added there had been little help for residents since disaster struck.

Most of the thousands of people displaced are staying in crowded shelters.