Indonesian authorities are on standby to divert flights destined for the holiday island of Bali as increasingly frequent tremors from a rumbling volcano stoke fears an eruption could be imminent.  
Mount Agung, 75 kilometres (47 miles) from the tourist hub of Kuta, has been shaking since August, threatening to erupt for the first time in more than 50 years and forcing more than 80,000 people to flee their homes.
Bali attracts millions of foreign visitors every year to its palm-fringed beaches and an eruption would be a blow to its tourism-dependent economy.
The airport in Bali's capital Denpasar has not been affected, but several countries including Australia and Singapore have issued advisories warning travellers to exercise caution.
In the case of an eruption, Indonesia plans to divert flights headed for Bali to ten other airports, including on nearby Lombok and the capital Jakarta.
"The planes will be diverted to their nearest location or where it originally took off from," transport minister Budi Karya Sumadi said.
Airlines are watching the situation closely and 100 buses have been prepared to evacuate tourists.
Virgin Australia said it would be making an extra fuel stop in Darwin for some of its flights between Australia and Bali in case it is forced to turn back.
Singapore Airlines said customers travelling between September 23 and October 2 could rebook flights or ask for a refund.
- Eruption jitters -
Officials announced the highest possible alert level on Friday due to the increasing volcanic activity, and told people to stay at least nine kilometres away from the crater.
They are still stressing the island is generally safe, but there are signs the volcano is starting to give tourists the jitters.
I Komang Nik Suantara, owner of the Amed Beach Resort on Bali’s east coast, said business had collapsed since the alert level was raised.
"Amed is safe from the lava but still the economy is very down, no customer, everyone cancelled,” he said.
German tourist Christoph Lange, who is staying in Amed, said that despite frequent tremors he felt "pretty safe".
"We've got like 20, 30, 40 shakes where you could feel the earth shaking," Lange, 30, told AFP.
"While we were diving we had quite a few. Under water you can hear it, it's pretty loud actually."
Rahmat, who sells clothes from a stall in Kuta, said his income had halved in the past week because there were fewer tourists.
The Indonesian Center for Volcanology and Geological Hazard Mitigation said tremors were increasing and it had recorded 480 between 12am and 12pm on Wednesday. A thin column of smoke can be seen rising from the mountain's summit.
"So far the activity of Mount Agung remains high, the alert level is on level 4 -- the highest," said Kasbani, head volcanologist at the centre who like many Indonesians has one name.
Indonesia lies on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent seismic and volcanic activity.
Mount Agung last erupted in 1963, killing nearly 1,600 people.