The alert for a volcano spewing huge billowing columns of ash and smoke into the air on Bali has been raised to the highest level, with fears a significant eruption could be imminent.

Authorities have widened the exclusion zone around Mount Agung to 6 miles (10km) and have told people living nearby the mountain to immediately evacuate.

"Plumes of smoke are occasionally accompanied by explosive eruptions and the sound of weak blasts that can be heard up to 12km (7m) from the peak," the Disaster Mitigation Agency said in a statement after raising the alert level from three to four.

"The potential for a larger eruption is imminent," it said.

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Timelapse video of Bali volcano spewing ash

The airport in Bali's capital, Denpasar, a top holiday destination that attracts millions of tourists every year, has been closed, causing around 2,000 passengers - mostly from Australia - to become stranded.

In response, the Australian government advised its nationals to "exercise a high degree of caution in Indonesia" and "follow the instructions of authorities".

Image: A 6-mile exclusion zone has been placed around Mount Agung

Plumes of ash and steam as high as 13,000ft (4,000 metres) have been pouring out of Mount Agung since Tuesday.

Gede Suantika, an official at the disaster mitigation agency, said: "The activity of Mount Agung has entered the magmatic eruption phase.

"It is still spewing ash at the moment, but we need to monitor and be cautious over the possibility of a strong, explosive eruption."

Image: Farmers living near the base of the volcano have had to flee

Mount Agung is one of more than 120 active volcanoes extending the length of Indonesia that sits on the so-called Pacific Ring of Fire.

It last erupted in 1963, killing about 1,600 people.

Mount Agung rumbled back to life in September and authorities raised the alert to the highest level, forcing 140,000 people to evacuate.

Image: People have fled to evacuation centres in Bali

The volcano's activity decreased in late October and many people returned to their home as the alert was lowered to the second-highest level.

But Mount Agung started rumbling again last week.