A volcano in Indonesia could remain on the brink of a major eruption for weeks as it continues to spew ash across the island of Bali, volcanologists have warned.

Mount Agung has been spitting ash 4,000m (2.5 miles) into the air, stranding tens of thousands of tourists as the international airport remained closed for a second day.

Lava has been welling inside the volcano’s crater, but it remained unclear how bad the eruption might get or how long it could last.

David Rothery, Professor of Planetary Geosciences at the Open University, told The Independent there was a 50-50 chance of a large eruption.

“It could stay like this and fizzle out, or it could be ramping up for something big,” he said.

Bali's Mount Agung volcano spews plume of ash 6,000 metres into the sky

Dr Matt Watson, from the University of Bristol, said: “The truth is it could peter out tomorrow. I don’t think that’s particularly likely, but it’s not impossible.

“The reason people are worried about a large eruption is because the last time it erupted it went large. It was a big enough eruption to influence global temperatures.”

Mount Agung’s last major eruption in 1963 killed about 1,100 people.

It has the potential to disrupt travel for weeks, Mr Rothery said. “They’re not going to get the airport running anytime soon. If it does what it did in 1963, it could go on for weeks.

“On the other hand, if it has a big eruption, it could be over in a week.”

Bali's Mount Agung volcano eruption: in pictures Show all 22 1 /22 Bali's Mount Agung volcano eruption: in pictures Bali's Mount Agung volcano eruption: in pictures A fisherman drives a traditional boat as Mount Agung erupts AFP/Getty Bali's Mount Agung volcano eruption: in pictures Police evacuate a villager from inside the exclusion zone REUTERS Bali's Mount Agung volcano eruption: in pictures Villagers watch a river overflowing with water mixed with volcanic ash REUTERS Bali's Mount Agung volcano eruption: in pictures A flight information board shows cancelled flights at Ngurah Rai International Airport in Denpasar, Bali AP Bali's Mount Agung volcano eruption: in pictures A villager takes his cows to a field with Mount Agung volcano erupting in the background AP Bali's Mount Agung volcano eruption: in pictures Police officers wear masks as they patrol in a village in Karangasem AP Bali's Mount Agung volcano eruption: in pictures Clouds of ashes rise from the Mount Agung volcano AP Bali's Mount Agung volcano eruption: in pictures Passengers talk to airline workers as their flights are cancelled AP Bali's Mount Agung volcano eruption: in pictures Mount Agung AP Bali's Mount Agung volcano eruption: in pictures A boy takes pictures during Mount Agung's eruption AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano is seen spewing smoke and ash in Bali EMILIO KUZMA-FLOYD/via REUTERS Bali's Mount Agung volcano eruption: in pictures Balinese Hindus take part in a ceremony, where they pray near Mount Agung in hope of preventing a volcanic eruption AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano erupts for a second time in less than a week REUTERS Bali's Mount Agung volcano eruption: in pictures Balinese Hindus take part in a ceremony AFP/Getty Images Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano is seen erupting from Glumpang village REUTERS Bali's Mount Agung volcano eruption: in pictures A Balinese farmer works as Mount Agung volcano spews volcanic ash EPA Bali's Mount Agung volcano eruption: in pictures Mount Agung during an eruption seen from Kubu sub-district in Karangasem Regency AFP Bali's Mount Agung volcano eruption: in pictures A villager walks near a rice field following the eruption of Mount Agung AP Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano spews volcanic ash Rex Features Bali's Mount Agung volcano eruption: in pictures An officer with the disaster management agency BPBD places a mask on child at a shelter for residents sheltering from Mount Agung volcano Antara Foto/Nyoman Budhiana/ via REUTERS Bali's Mount Agung volcano eruption: in pictures Mount Agung volcano erupts REUTERS Bali's Mount Agung volcano eruption: in pictures Mount Agung's eruption is seen between Balinese temple at Kubu AFP/Getty Images

Authorities raised the volcano alert for Mount Agung to its highest level and told 100,000 people to leave an area extending 10km (6 miles) from its crater.

Indonesia’s National Disaster Mitigation Agency has said a larger eruption is possible, though a government volcanologist has also said Mount Agung could stay at its current level of activity for weeks and not erupt explosively.

However, a major eruption has the potential to damage crops on the island and kill anyone within the 10km evacuation radius.

A villager takes his cows to a field with Mount Agung volcano erupting in the background (AP)

“It depends on how well the evacuation zone is applied,” Dr Watson said. “Volcanoes on long fuses give authorities an advantage.

“They allow people to carefully and thoughtfully manage the process. If there’s nobody in the exclusion zone, most of the really dangerous hazards will not affect many people.”

But a large eruption could destroy the island’s agriculture and have a major impact on tourism, he added.

A fisherman drives a traditional boat as Mount Agung erupts (Getty) (AFP/Getty)

A Nasa satellite detected a thermal anomaly at the crater, senior Indonesian volcanologist Gede Swantika told the Associated Press, meaning a pathway from the storage chamber in the volcano’s crust has opened, giving magma easier access to the surface.

Indonesian officials first raised the highest alert two months ago when a rash of seismic activity was detected at the mountain.

More than 100,000 people living near the volcano fled their homes, many abandoning their livestock or selling them for a fraction of the normal price. The seismic activity decreased by the end of October, causing authorities to lower the alert level.

Tremors increased again last week and officials upped the alert and ordered another large-scale evacuation, with nearly 40,000 people now staying in 225 shelters, according to the Disaster Mitigation Agency in Karangasem.