Karangasem, Indonesia: A rumbling volcano on Bali could erupt at any moment, authorities warned Monday as they raised alert levels to maximum, accelerated a mass evacuation and closed the main airport, leaving thousands of tourists stranded on the Indonesian resort island.

Massive columns of thick grey smoke that have been belching from Mount Agung since last week have now begun shooting more than three kilometres (two miles) into the sky, forcing hundreds of flights to be grounded.

Some 40,000 frightened people have fled their homes around the volcano but as many as 100,000 will likely be forced to leave, disaster agency officials said, after raising the alert to its highest level.

The exclusion zone around Agung, which is 75 kilometres (47 miles) from the beachside tourist hub of Kuta, has also been widened to 10 kilometres.

Makeshift tents and community centres filled up Monday as nearly two dozen villages were emptied of their inhabitants, including farmers reluctant to leave precious livestock behind.

"Continuous ash puffs are sometimes accompanied by explosive eruptions and a weak booming sound," the National Board for Disaster Management said earlier Monday.

"The rays of fire are increasingly observed at night. This indicates the potential for a larger eruption is imminent."

Agung rumbled back to life in September, forcing the evacuation of 140,000 people living nearby. Its activity decreased in late October and many returned to their homes.

However, on Saturday the mountain sent smoke up into the air for the second time in a week in what volcanologists call a phreatic eruption -- caused by the heating and expansion of groundwater.

Then on Monday so-called cold lava flows appeared -- similar to mud flows and often a prelude to the blazing orange lava seen in many volcanic eruptions.

"I'm very concerned because I left my house behind and I'm also worried about family," said 36-year-old farmer Putu Suyasa, who fled with some relatives from a village eight kilometres away from the volcano.

"The mountain is spewing thicker smoke than before."

Scary memories

Dewa Gede Subagia was a teenager when Agung last roared.

"I am very worried because I have experienced this before," the now 67-year-old told AFP from one evacuation centre.

"I hope this time I won't have to evacuate for too long. In 1963, I left for four months."

Mt. Agung last went off in 1963, killing around 1,600 people in one of the deadliest eruptions ever seen in a country with nearly 130 active volcanoes.

The airport in Bali's capital Denpasar, a top holiday destination that attracts millions of foreign tourists every year, has been closed.

Some 445 flights were cancelled, affecting more than 59,000 passengers, officials said.

Colin Cavy, a French dive-master who has been in Indonesia for a couple of months, was nervously looking at his now-expired visitor visa.

"I need to go to the immigration office," he said.

While there was dismay from some tourists who were unable to return to their homes and jobs, others took events in their stride.

"What can I say? We have to cooperate because this is a natural disaster," said Indian visitor Krisna Mustafa.

Many were told that it could be several days before they could leave.

"My 7:00 am flight this morning got cancelled, just when we were about to board," said 23-year-old Indonesian tourist Merry Handayani Tumanggor.

"Now we have to stay in Bali again -- the earliest we can go is on Friday, they say."

The airport on nearby Lombok island east of Bali, also a popular tourist destination, was closed for a night on Sunday but reopened on Monday morning.

Officials announced late Monday that Lombok airport has been closed again until Tuesday morning due to the volcanic ash and at least 47 flights to and from the island have been cancelled.

The Australian government put out a travel advisory on Sunday, warning that volcanic activity "may escalate with little or no notice".

"Past eruptions of Mount Agung have shown this volcano's potential to cause significant impacts," it added.-APP