The number of people evacuated over the expected eruption of the Mt Agung volcano in the Indonesian island of Bali, the country's major tourist destination, is now more than 144,000.

Those evacuated have been housed in hundreds of shelters across nine districts on the island.

Sutopo Purwo Nugroho of the Indonesian National Board for Disaster Management (BNPB) that although the exclusion zone established around the volcano is now practically empty, there are still a few residents who have refused to leave, unwilling to abandon their livestock.

A 12-kilometre radius exclusion zone has been set up around the volcano and the alert level for possible eruption has been cranked up to the maximum 4.

"If an eruption does occur, it is most likely to be small at first; however, a large eruption may follow," head of Centre for Volcanology and Geological Hazard Mitigation (CVMPG), Kasbani, said in a statement.

Over the last few days a column of smoke has been emanating from the crater, reaching an altitude of between 50 and 200 metres.

CVMPG as well as the provincial government of Bali maintain it is safe to travel to the island, and a plan has been prepared to divert planes bound for Bali's Ngurah Rai International Airport in case of an eruption.

Experts say it is impossible to predict if the 3,031 metre high volcano will erupt or not, but that there is a greater possibility of an eruption.

The BNPB had said in a statement on September 21 the current tremors are similar to those that occurred before Mt Agung erupted in 1963, which lasted almost a year and killed over 1,100 people.

Indonesia is home to more than 400 volcanoes, of which at least 127 are active and 65 are classified as dangerous.

The Indonesian archipelago sits on the so-called "Pacific Ring of Fire", an area known for its great seismic and volcanic activities that produce thousands of earthquakes - mostly of small magnitudes - every year.