PARIS (AFP) - Indonesian authorities have raised the highest alert for Mount Agung on Bali - an island that attracted nearly five million tourists last year.
Authorities have widened the exclusion zone to a 10km radius, and have ordered people in the area to evacuate.
Mount Agung, Bali's highest peak, rumbled back to life in September, forcing the evacuation of 140,000 people living nearby. Its activity decreased in late October and many returned to their homes.
Officials said on Monday (Nov 27) potentially greater eruptions are very imminent. The so-called cold lava flows - similar to mud flows and often a prelude to the blazing orange lava seen in full-blown volcanic eruptions - also appeared on Monday.
Some experts warn that Mount Agung might at last be delivering the large full-blown eruption that has been feared for several weeks.
Others, however, are more cautious. They say volcanic eruptions are difficult to predict and it is very hard to tell exactly how the situation would develop.
In the event of a major eruption, the impact on Bali's multi-billion-dollar tourism industry would depend on how much of the island is blanketed in ash, along with the force and persistence of the blast.
Here's what experts are saying:
Things could get worse
"What we are seeing at the moment are small explosions, throwing out hot gases and fragments of molten rock, or ash," explained David Pyle, a volcano expert at the University of Oxford in Britain.
"The Agung volcano commenced a sustained ash eruption on Saturday, with plumes reaching 3,000 m high," explained Mark Tingay, a geologist at the University of Adelaide in Australia.
"The eruption has now moved on to the next, more severe phase, where viscous lava can trap gasses under pressure, potentially leading to an explosion."
A major eruption looks inevitable
Several scientists remarked that Agung's recent behaviour matches the build-up to the devastating 1963 blast that left 1,600 people dead and ejected enough debris - about a billion metric tonnes - to lower global average temperatures a notch (0.2 - 0.3 deg C) for about a year.
"Based on what we saw in 1963, the present activity is quite similar to the start of that eruption," said Pyle.
"The probability of a large eruption is high, but this may take some days or weeks to unfold."
David Rothery, a professor at The Open University in Britain, also sees a step-change on the horizon.
"The volcano might at last be delivering the large eruption that has been feared for several weeks," he said.
Jacques-Marie Bardintzeff, a volcanologist at Paris-Sud University, said that "all the warning lights are red."
"My Indonesian colleagues and I think that Agung will erupt," he told AFP.
Difficult to predict the situation
"We are still far from being able to forecast how eruptions are going to develop," said Carmen Solana, a volcanologist at the University of Portsmouth in England.
"It could rapidly increase in activity and produce a vast eruption, or it could die down." .
Worst case scenario
"The worst case scenario would be a repeat of the 1963 eruption, perhaps a little be larger," Pyle told AFP.
"The main areas that will need to be evacuated are 10-12 km from the volcano," he said. "There won't be a need for the whole island to be evacuated."
Mark Tingay of the University of Adelaide told the BBC News that Indonesian authorities appeared "extremely well prepared", with the situation "well under control".
A big blast would also produce "hot rock avalanches" down the flanks of the volcano, said Mike Burton, a professor at the University of Manchester in England.