The number of tremors recorded at Bali's Mt Agung volcano increased again late yesterday after a lull during the morning.

Authorities say they still cannot predict when the mountain will erupt but with the threat level at its highest, an emergency response period has been declared.

And Bali's Ngurah Rai airport (Denpasar International Airport) is preparing an emergency operations centre, in the event of an eruption closing the busy airport.

Villagers rest at a temporary shelter in Klungkung, Bali, Indonesia. Photo / AP
Villagers rest at a temporary shelter in Klungkung, Bali, Indonesia. Photo / AP

The Operations Section Head of I Gusti Ngurah Rai Airport, Misranedi, said yesterday that airports in Lombok and Surabaya, in East Java, were being prepared as alternatives, assuming they too were not affected.

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Misranedi said that taxis and buses would be provided to take people to the Padangbai seaport so they could go by boat to Lombok or Surabaya.

"We are preparing our emergency operation centre (EOC). In the EOC, we will co-ordinate with the BMKG, Air Navigation, and all related parties. When the eruption happens and the airport must be closed, we have prepared alternative airports, such as Lombok airport and Juanda airport. I hope the two airports will not be affected by the eruption," Misranedi said.

According to police almost 28,000 villagers living near Mt Agung volcano have now been evacuated to shelters.

Hundreds of tremors, from deep within Mt Agung, are now being recorded daily as the majestic mountain rumbles into action for the first time in five decades.

Mount Agung volcano is seen at the sunrise in Bali, Indonesia. Photo / AP
Mount Agung volcano is seen at the sunrise in Bali, Indonesia. Photo / AP

In the 12 hours from midnight on Friday until noon yesterday, a total of 198 tremors were recorded.

The threat level was increased to four on Friday night, by the Centre for Volcanology and Geological Hazard Mitigation agency, the third time in the past week the level has been raised.

And the exclusion zone was doubled to 12km from the summit, a calculation based on the track of ash cloud and lava from the last time Mt Agung erupted, back in 1963, when 1100 people were killed.

The head of the Centre for Volcanology and Geological Hazard Mitigation (PVMBG), Kasbani, said yesterday that very small tremors had been detected at Mt Agung since its last eruption in 1963. These started to increase markedly last month and this month had reached an extreme level.

"Four days after we raised the alert level to level three, (earlier this week) there were extraordinary tremors ... the biggest since 1963. So, we raised the alert level to level four," Kasbani said.

Early yesterday the tremors had started to decrease but by the afternoon were increasing again.

"We could not predict when the mountain will erupt," he said.

Nor could they predict how long the eruption will last. But based on the 1963 eruption, it could be erupting for a year.

"However, we don't know whether the eruption now will be bigger or smaller. If we see the eruption in 1963, it could take one year," Kasbani said.

He said the 1963 eruption had seen hot ash clouds gush out with extraordinary speed, reaching 14km to the north, 12km to the southeast and 12km to the south and southwest.

At that time, lava and rocks the size of a human head had rained down.

The National Disaster Mitigation Agency (BNPB) head, Willem Rampangilei, said all people in the region 9-12km from the mountain must evacuate.

A man observes a map of the Mount Agung volcano at the Mount Agung monitoring station in Bali, Indonesia. Photo / AP
A man observes a map of the Mount Agung volcano at the Mount Agung monitoring station in Bali, Indonesia. Photo / AP

"We have prepared 500,000 masks to anticipate volcanic ash which is very important, because the ash is very dangerous," Rampangilei said yesterday.

"This is very complex work. We should work hard to minimise victims. We keep hoping that the eruption will not happen. However, we should be ready for the best scenario if the eruption does happen," he said.

"We have declared that we are in emergency response period for [the] next one month. I hope, the eruption will not happen."

TRAVEL WARNING

DFAT has updated its travel advisory, warning tourists to monitor the situation closely and follow instructions of officials, saying an eruption could impact air travel.

Bali tourist officials have also become frustrated at exaggerated reporting causing panic among tourists. Mt Agung is about 72km from the densely populated tourist district of Kuta and concern is highest for those locals living in the villages surrounding the mountain itself. The main concern for tourists is the expected closure of the airport and delayed flights should the mountain erupt.

ANIMALS FLEE

Indonesian media is reporting that wild animals - including snakes and apes - are panicked by the stirring volcano and moving through settled areas.

It's been a growing trend over the past three days, the Tribun Kaltim news service says.

"It may be hot on Mt Agung. So the animals [come] out and to the settlement," it reports district identity Jro Mangku as saying.

Men from a traditional village in the volcano's shadow believe the descent of animals from the top of the mountain is one of "seven signs" an eruption will occur. Small numbers begin to move up to three months before an eruption.

"Maybe this is a sign - the sign of the mountain will erupt. This condition is not as usual," Jro Mangku reportedly said.

Women sit in their temporary shelter in Bali, Indonesia. Photo / AP
Women sit in their temporary shelter in Bali, Indonesia. Photo / AP

Such an animal exodus was observed before Mt Agung's previous eruption in 1963.

Other signs locals have come to expect before an eruption are yet to emerge. For example, there is no evidence of fine ash yet, which can cause skin to itch.

GROWING HAZARD

The Volcano Observatory Notice for Aviation reports the volcano's seismic activity has dramatically increased. "This number of seismicity is an unprecedented seismic observation at Agung volcano ever recorded by our seismic networks," it said in a statement.

Earlier, the Department of Meteorology, Climate and Geophysics said in a statement there has been a "tremendous increase" in seismic activity at the mountain, indicating a greater probability of an eruption.

Children stay in a truck as their temporary shelter in Klungkung, Bali, Indonesia. Photo / AP
Children stay in a truck as their temporary shelter in Klungkung, Bali, Indonesia. Photo / AP

Indonesia's National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho said overnight Friday that the hazardous zone had been increased from 9km to 12km, covering an area encompassing about 240,000 people and prompting further evacuations. He urged people to "calm down" and seek reliable information.

"Estimated danger zones are dynamic and are being continuously evaluated. [They] are subject to change at any time following the most recent observation data," Indonesia's volcano observation authority warned.

'KILLER' VOLCANO

Indonesia's volcano monitoring body, MAGMA, warns Mt Agung's eruptions are characteristically explosive and effusive - resulting in deadly pyroclastic flows of ash, rock and lava.

"In case of eruption, the potential primary hazard that may occur within a radius of 9km is pyroclastic fall of size equal to or greater than 6cm," its website states.

But its modelling for some of the terrain around the volcano also shows such flows could cover 10km in less than 3 minutes.

"If an eruption occurs, there is considerable disaster potential," it warns. "People around Mt Agung also do not have enough experience to face the eruption because this volcano last erupted ... 54 years ago."

Agung last erupted in 1963, unleashing deadly pyroclastic flows which killed about 1100 people and hurling ash as high as 10km.

Villagers who evacuated from their homes on the slope of Mount Agung sit in a truck upon arrival at a temporary shelter in Klungkung, Bali, Indonesia. Photo / AP
Villagers who evacuated from their homes on the slope of Mount Agung sit in a truck upon arrival at a temporary shelter in Klungkung, Bali, Indonesia. Photo / AP

It is just one of 130 active volcanoes in Indonesia, part of the Pacific "Ring of Fire" convergence of tectonic plates.

Emeritus professor Richard John Arculus of Australian National University has published a blog saying Mt Agung has produced some of the largest eruptions of the past 100 years.

"Our ability to predict eruptions has improved dramatically since this last event, so we can hope such a death toll will not occur again," he writes.

"A primary line of evidence is the frequency and locations of earthquakes beneath the volcano, caused by upward-flowing magma. Swelling and inflation of the volcano coupled with measurements of the temperatures and composition of gases emerging from the crater also give clues as to the likelihood of an eruption.

"So there is no need to be caught unawares by Mt Agung, providing the advice of the authorities, armed with expert assessments, is followed."