The impacts of Bali’s erupting volcano have spread beyond a “danger zone” defined by the Indonesian authorities, as 100,000 people were ordered to flee on Monday and the island’s international airport was forced to close.

Ash from Mount Agung has been spewing up to about 3,000m (9,800ft) into the atmosphere, and experts say the immediate area will be prone to deadly mudflows and lava while the eruption continues.

Video released by the National Disaster Mitigation Agency showed an avalanche of volcanic debris and water known as a lahar moving down the side of the mountain.

The agency raised the volcano’s alert to the highest level early on Monday and expanded the danger zone to 10km (6 miles) in places from the previous 7.5km. It said in a statement that a larger eruption is possible.

Spokesperson Sutopo Purwo Nugroho told a news conference in Jakarta that the extension of the danger zone affects 22 villages and about 90,000 to 100,000 people. He said about 40,000 people have evacuated but others have not left because they feel safe or don’t want to abandon their livestock.

“Authorities will comb the area to persuade them,” he said. “If needed we will forcibly evacuate them.” About 25,000 people were already living in evacuation centres after an increase in tremors from the mountain in September sparked an evacuation.

Bali’s airport was closed early Monday after ash reached its airspace. Almost 60,000 travellers were stranded by the cancellation of 445 flights, a spokesperson said.

Flight information boards showed rows of cancellations as tourists arrived at the busy airport expecting to catch flights home. The closure is in effect until Tuesday morning, though officials said the situation will be reviewed every six hours.

Bali is Indonesia’s top tourist destination, with its gentle Hindu culture, surf beaches and lush green interior attracting about 5 million visitors a year.

Mount Agung volcano – in pictures Show all 8 1 /8 Mount Agung volcano – in pictures Mount Agung volcano – in pictures The Mount Agung volcano spews hot volcanic ash, as seen from Datah, Karangasem, Bali, Indonesia. According to media reports, the Indonesian national board for disaster management raised the alert for the Mount Agung volcano to the highest status and closed the Ngurah Rai International Airport in Bali due to the ash cloud rising from the volcano EPA Mount Agung volcano – in pictures A long exposure photograph shows the Mount Agung volcano spewing hot volcanic ash as seen from Amed, Karangasem regency, Bali, Indonesia. EPA Mount Agung volcano – in pictures A villager walks as Mount Agung volcano erupts in the background in Kubu, Karangasem, Bali, Indonesia Reuters Mount Agung volcano – in pictures Women wait for customers at a market as smoke rises from the Mount Agung volcano AP Mount Agung volcano – in pictures Vegetable sellers wait for customers at a market with Mount Agung erupting in the background AP Mount Agung volcano – in pictures Balinese people ride on an open car past Mount Agung erupting seen from Kubu sub-district in Karangasem Regency AFP/Getty Mount Agung volcano – in pictures Villagers rest at a makeshift tent at an evacuated area in Karangasem AP Mount Agung volcano – in pictures Evacuees stay at an emergency shelter in Klungkung, Bali EPA

Some flights to and from Bali were cancelled on Saturday and Sunday but most had continued to operate normally as the towering ash clouds were moving east toward the neighbouring island of Lombok.

On Monday, however, the winds changed, with weather experts warning ash would be blown south-west towards the airport, as well as the island’s main tourist beaches and beyond to Java.

“We now have to find a hotel and spend more of our money that they’re not going to cover us for when we get home unfortunately,” said Canadian tourist Brandon Olsen, who was stranded at Bali’s airport with his girlfriend.

Indonesia’s Directorate General of Land Transportation said 100 buses are being deployed to Bali’s international airport and to ferry terminals to help travellers stranded by the eruption of Mount Agung.

The agency’s chief, Budi Setyadi, said major ferry crossing points have been advised to prepare for a surge in passengers and vehicles. Stranded tourists could leave Bali by taking a ferry to neighbouring Java and then travel by land to the nearest airports.

The geological agency head, Kasbani, who goes by one name, said the alert level was raised because the volcano has shifted from steam-based eruptions to magmatic eruptions. He told Indonesian television on Monday morning that he did not expect a big eruption but added: “We have to stay alert and anticipate.” The volcano’s last major eruption in 1963 killed about 1,100 people.

UK tourists stranded in Bali are being advised to avoid exclusion zones. The Foreign and Commonwealth Office also urged holidaymakers to contact their travel company and monitor local media reports in an update to its travel advice.

For the inhabitants of the island, the impacts are likely to be far more longstanding.

At Buana Giri village, some residents said they were leaving because the area is now inhospitable for their livestock.