Increasingly frequent tremors show that the molten magma is still rising towards the surface, with the mountain entering a "critical phase", said the national disaster mitigation agency.

It said the number fleeing their homes had increased as fears grow that the mountain could blow. "The local mitigation agency reported that until 12 pm Tuesday, the number has reached 75,673 people, spread across 377 evacuation centres in nine districts," said agency spokesman Sutopo Purwo Nugroho. Evacuees are taking shelter at temporary camps, sport centers, village halls and the houses of friends and relatives.

Villager Wayan Merta said he was among the first to evacuate last week because his village, Selat, is just 6 kilometers (4 miles) from the summit. "We have already sold our cattle, because we thought it was better than leaving them there for nothing," he said. "My feeling is the mountain will erupt," he said. "But no one knows, we just pray." Some 2,000 cows have also been evacuated from the flanks of the volcano.

President Joko Widodo was due to visit crammed evacuation centres in Bali on Tuesday afternoon. Balinese residents, international NGOs and the central government have begun organising aid.

The airport in Bali's capital Denpasar, through which millions of foreign tourists pass every year, has not been affected, but several countries including Australia and Singapore have issued a travel advisory. Flights to and from the island have not been interrupted but airlines are watching the situation closely.

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.

The volcano's alert status was raised to its highest level on Friday, with hundreds of tremors daily indicating a high chance it will erupt.

The 3,030-metre Mount Agung last erupted in 1963-64, killing about 1,500 people.  

The volcano is popular among local and foreign trekkers. Indonesia sits on the Pacific Ring of Fire, an area known for frequent seismic upheavals and volcanic eruptions. The country is home to about 130 active volcanoes.

is/at/ch (afp, ap, dpa)