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By Sarah Harman, Linda Givetash and Gabe Joselow

CILEGON, Indonesia — The streets of this city were still bustling Thursday as officials widened the danger zone and raised the threat level from an island volcano that continued to seethe after triggering a tsunami that killed more than 430 people.

Officials issued a level three tsunami alert — the second highest on its four-level system — and rerouted flights as emergency crews still searched for victims from last weekend's unexpected wave.

Anak Krakatau's eruption on Saturday evening caused part of the island in the Sunda Strait to collapse into the sea, generating massive tsunami waves.

The country's disaster mitigation center doubled the at-risk area to a 3,200-foot stretch area of the coast on Thursday and warned people to stay away from the water's edge because of the risk of another tsunami triggered by Krakatau's ongoing eruptions.

Faisal Muhammad Kais told NBC News he was keeping a close eye on his phone for evacuation alerts from officials.

"I'm so worried because we are very close to the sea," he said.

Cilegon sits west of the capital and about 3 miles from the coast on the island of Java which, along with Sumatra Island, bore the brunt of the tsunami.

Aircraft were forced to take alternate routes Thursday to avoid the increasing volcanic ash mushrooming up from Anak Krakatau, said Yohanes Sirait, spokesman for AirNav Indonesia. Most scheduled flights through the capital of Jakarta have not been affected.

Although ash billowed more than 6 miles into the skies, airports integral to the distribution of aid and resources to areas affected by the tsunami were not forced to close.

"There's still a chance of a landslide, even under the sea level or on the sea level," said Rudy Sunendar, head of the energy ministry's geology department. "Based on the satellite imagery interpretation, there is collapse of some area of Mount Anak Krakatau."HANDOUT / AFP - Getty Images

The cloud of ash and sand rained down on Cilegon, coating parked cars and streets with a thin layer of dust. People were warned to stay indoors and those who had to be outside were urged to wear masks and goggles, Indonesia's disaster mitigation center said.

The conditions were concerning for parents with small children. "I'm worried about my son's respiration," Iput Salima told NBC News about her four-year-old son.

Not everyone had time to make preparations. Lia Amalia, 20, who works at a hijab boutique, said she forgot to buy a mask but was ready to go to higher ground if a tsunami hit.

Ships and recreational vessels in the Sunda Strait were warned to keep a safe distance from the island of Krakatau and to be prepared for extreme conditions.

A tsunami hit the coast Saturday after a 158-acre section of rock from Anak Krakatau's southwest side — the size of nearly 120 football fields — collapsed into the ocean. The shock, equivalent to a 3.4-magnitude earthquake, triggered the wave that hit land 24 minutes later.

Saturday's disaster struck without warning, taking people by surprise in a country that regularly suffers landslides, earthquakes and volcanic eruptions. No big earthquake shook the ground beforehand, and the waves surged inland at night on a holiday weekend while people were enjoying concerts and other beach and resort activities.

The volcano has been spewing ash and lava for months.

There is no warning system in place for volcano-triggered tsunamis. Existing systems are based on monitoring earthquakes, which officials have said account for 90 percent of tsunamis in the region.

Dozens remain missing from Saturday's disaster and thousands have been displaced. A state of emergency remains in effect until Jan. 4 to help distribute aid.

Sarah Harman and Gabe Joselow reported from Cilegon, with Linda Givetash from London.

Reuters and Associated Press contributed.