Indonesia has raised its warning for Bali's Mount Agung volcano to the top level, closed the holiday island's airport and told residents around the mountain to immediately evacuate, warning of an "imminent" risk of a larger eruption.
Bali's airport was closed for 24 hours from Monday morning, disrupting 445 flights and some 59,000 passengers, due to the eruption and the presence of volcanic ash from Agung, but local officials said the closure could be extended.
Video footage shared by the disaster agency showed cold lava flows (lahar) at a number of locations on the mountainside. Lahar carrying mud and large boulders can destroy houses, bridges and roads in its path.
"Plumes of smoke are occasionally accompanied by explosive eruptions and the sound of weak blasts that can be heard up to 12km from the peak," the Disaster Mitigation Agency (BNPB) said in a statement after raising the alert level from three to four.
"The potential for a larger eruption is imminent," it said, referring to the visible glow of magma at Agung's peak overnight.
Residents were warned to "immediately evacuate" a danger zone that circles Agung in a radius of 8-10km.
Sutopo, a BNPB spokesman, said there had been no casualties so far and 40,000 people had left the area, but tens of thousands still needed to move and warned authorities would move them by force if necessary.
Agung rises majestically over eastern Bali to a height of just over 3,000m. The last eruption in 1963 left more than 1,000 people dead and razed several villages.
According to the Volcanic Ash Advisory Centre in nearby Darwin, Australia, there is "ash confirmed on the ground at Denpasar Airport" as well as ash at FL300 (which refers to flight level at 30,000 feet) in the vicinity of the volcano.
Bali's I Gusti Ngurah Rai airport, which is about 60km from the volcano, will be closed for 24 hours, according to its operator. A total of 445 flights - 196 international and 249 domestic - and 59,000 passengers had been affected.
Ten alternative airports have been prepared for airlines to divert inbound flights, including in neighbouring provinces.
The airport operator said it was providing buses to take travellers to ferry ports for alternative travel arrangements.
Bali airport's official website showed flights operated by Singapore Airlines, Sriwijaya, Garuda Indonesia , Malaysia Airlines and Jetstar had been cancelled.
Television footage showed hundreds of holidaymakers camped inside the airport terminal, some sleeping on their bags, others using mobile telephones.
Cover-More, Australia's biggest travel insurer, said on its website customers would only be covered if they had bought policies before the volcano alert was first issue on September 18.