Bali on fire: Mt Agung volcano alert level raised, thousands flee

Indonesia today raised its warning for Bali's Mount Agung volcano to the top level four alert on Monday, closed the holiday island's airport and told residents near the summit to immediately evacuate, warning of the ''imminent'' risk of a larger eruption.

Massive columns of thick grey smoke that have been belching from Mount Agung since last week have now begun shooting more than 3 km into the sky, forcing 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 km from the beachside tourist hub of Kuta, has also been widened to 10 km.

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

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

Bali's airport was closed for 24 hours, disrupting 445 flights and some 59,000 passengers, due to the eruption warning and the presence of volcanic ash from Agung.

TV footage showed cold lava flows (lahar) at a number of locations on the mountainside. Lahar carrying mud and large boulders can destroy houses, bridges and roads in its path.

Agung rises majestically over eastern Bali to a height of just over 3,000 metres (9,800 feet). Agung's last eruption in 1963 left more than 1,000 people dead and razed several villages.

Bali, famous for its surf, beaches and temples, attracted nearly 5 million visitors last year, and its international airport serves as a transport hub for the chain of islands in Indonesia's eastern archipelago.

Tourism business has slumped in Bali since September when Agung's volcanic tremors began to increase.

''In anticipation of the possibility and imminent risk of disaster,'' PVMBG (the Volcanology and Geological Disaster Mitigation Centre) said in a statement it had ''raised Mount Agung alert level from three to four.''

According to the Volcanic Ash Advisory Centre in nearby Darwin, Australia, there is ''ash confirmed on the ground at Denpasar Airport'' as well as ash at FL300 (which refers to flight level at 30,000 feet) in the vicinity of the volcano.

Bali's I Gusti Ngurah Rai airport, which is about 60 km (40 miles) from the volcano, will be closed for 24 hours, according to a statement from its operator. It said a total of 445 flights - 196 international and 249 domestic - and 59,000 passengers had been affected.

Five alternative airports have been prepared for airlines to divert inbound flights, such as airports in neighbouring provinces.

Bali airport's official website showed flights operated by Singapore Airlines, Sriwijaya, Garuda Indonesia , Malaysia Airlines and Jetstar had been cancelled.

Television footage showed hundreds of holidaymakers camped inside the airport terminal, some sleeping on their bags, others using mobile telephones.

A transport ministry spokesman said the main airport on the neighbouring island of Lombok had reopened after ''no volcanic ash was detected".

Airlines avoid flying when volcanic ash is present because it can cause significant damage to aircraft engines, leading to possible engine failure, and can clog fuel and cooling systems and hamper pilot visibility.

In June 1982, a British Airways 747 suffered severe damage and had all four engines flameout after encountering ash from Mount Galunggung in Indonesia. It descended to 12,000 feet before being able to restart some engines and make an emergency landing in Jakarta.

The alert level on Agung had been raised to the maximum in September, but was lowered in October when seismic activity calmed. However, volcanologists now say the volcano has entered a new phase with magma now visible and meaning a greater risk of a large eruption.

Indonesia is the world's most active volcanic region. The archipelago nation with over 17,000 islands lies on the Pacific "Ring of Fire" where tectonic plates collide, causing frequent volcanic and seismic activities.

Last year, seven were killed after Mt Sinabung on the western island of Sumatra erupted, while 16 were left dead by a Sinabung eruption in 2014.