A rumbling volcano on the holiday island of Bali is spewing steam and sulfurous fumes with more intensity, heightening fears of an eruption as officials said the number of evacuees had topped 144,000.

Mount Agung, 75 kilometers from the resort hub of Kuta, has been shaking since August and threatening to erupt for the first time since 1963 - a potential blow to the island's lucrative tourism industry.

The Indonesian Center for Volcanology and Geological Hazard Mitigation said on Friday that remote satellite sensing had picked up new steam emissions and thermal areas within the crater.

White steam clouds - which contain sulfurous fumes - have been observed rising 50 to 200 meters above the summit, the center said. "At this moment, the probability of an eruption is higher than the probability of no eruption; however, the probability may change," said Kasbani, the centre's head volcanologist, who like many Indonesians goes by one name.

Another of the volcanologists at the center, Gede Suandika, said the more frequent emission of sulfurous fumes in the past three days indicated the volcano was changing.

"This morning the steam billowed from the crater like the smoke that comes out of a factory chimney," he said.

"Since the sulfurous fumes are out, the possibility of an eruption is getting more real."

Elsewhere, the threat of a separate volcanic eruption on the Pacific archipelago of Vanuatu prompted authorities on Thursday to order the compulsory evacuation of the entire island of Ambae, home to 11,000 people.

Vanuatu's National Disaster Management Office said it was the first time in living memory that an island's whole population had been moved because of volcanic activity.

Plumes of smoke, ash and volcanic rocks have been filmed erupting from its crater by New Zealand Defence Force aircraft which flew over the island on Tuesday.

The alert level for the erupting volcano was raised to four in recent days - level five is the highest - and Vanuatu's Meteorology and Geohazards Department warned of flying rocks, volcanic gas, acid rain and ash falls.