Around 100,000 people near Bali’s Mount Agung have been ordered to evacuate as officials fear a major eruption.
Indonesian authorities have raised the state of alert to its highest level, and expanded the exclusion zone around the rumbling volcano.
The island’s airport has now closed, leaving thousands stranded in the tourist hotspot.
Authorities say dark smoke and ash have been billowing up to 3,400m (11,150ft) above the mountain’s summit.
Officials have warned residents to stay away from rock and debris flows known as lahars, which have been spotted flowing down from the mountain.
The National Board for Disaster Management raised the alert to level four from 06:00 local time (22:00 GMT Sunday), because of “the possibility and imminent risk of disaster”.
The volcano was seen emitting “continuous ash puffs” occasionally accompanied by “explosive eruptions” and “weak booms” that could be heard 12km (7 miles) away from the summit.
“The rays of fire are increasingly observed at night. This indicates the potential for a larger eruption is imminent,” it said in a statement (in Indonesian) on its Facebook page.
Geologist Mark Tingay of the University of Adelaide said that Mount Agung now appears to be entering the next phase with a magmatic eruption, where the glow of lava is visible from the crater.
But he added that as eruptions are difficult to predict, it was “very hard to tell” how the situation would develop. “These eruptions can be quite large and extensive and violent, but this could also be a relatively minor eruption over time.”
He added that Indonesian authorities had made preparations for an eruption for months, and said they appeared “extremely well prepared”, with the situation “well under control”.
The main tourist stretch of Kuta and Seminyak is about 70km (43 miles) from the volcano, far beyond the exclusion zone.
But the ash prompted officials to shut down Bali’s Ngurah Rai airport from Monday to Tuesday morning.
Airport authorities said 445 flights had been cancelled, affecting 59,000 travellers, while Indonesia’s national hotel association said that stranded tourists staying at member hotels could stay one night for free.
The Australian government and the UK’s Foreign Office have advised travellers to be prepared for cancelled flights, closely monitor local media, and to heed local authorities’ warnings.
China (statement in Chinese) advised its citizens to be “cautious” if travelling to Bali.
The airport on neighbouring island Lombok reopened on Monday morning after it was shut briefly.
Authorities have widened the exclusion zone to a 10km (6 mile) radius from the volcano, and have ordered people in the area to evacuate.
On Monday, officials said about 100,000 residents needed to leave, but so far only 40,000 have done so.
The volcano first began belching thick smoke last week – its first eruption in 50 years.
Officials have also been distributing masks for local residents, as ash rains down in the vicinity.
Anna Baranova, who works with non-governmental organization Kopernik that is helping evacuees, said her group was trying to bring in better masks and possibly goggles.
Lahars, also known as “cold lava”, are slurries of rock fragments mixed with water, and have been spotted in fields and rivers near the volcano.
The spokesman for the national disaster management agency, Sutopo Purwo Nugroho, has warned that lahars may increase due to heavy rain and as Mount Agung continues to emit pyroclastic material.
He has urged islanders to stay away from lahars as well as avoid activities near rivers.
Authorities first issued warnings of an imminent eruption and raised the alert to the highest level in September after detecting heightened volcanic activity, prompting mass evacuations.
But some islanders returned to their homes in October after the alert level was lowered with a decrease in activity.
According to official estimates, the holiday island lost at least $110m (£83m) in tourism and productivity during that major evacuation.
The last time Mount Agung erupted, in 1963, more than 1,000 people died.
Indonesia sits on the Pacific “Ring of Fire” where tectonic plates collide, causing frequent seismic and volcanic activity. It is home to more than 130 active volcanoes.