Mount Kelut, a 1,731-metre (5,712-foot) volcano with a history of deadly eruptions, was put on high alert on October 16, triggering efforts to evacuate about 130,000 people living within 10 kilometres (six miles).

A scientist at Kelut's monitoring post said water in the volcano's 15-metre deep crater reached 39 degrees Celsius in the first six hours of Saturday, just below the 40 degrees reached before the 1990 eruption, he said.

"The increasing temperature shows that the magmatic pressure within the volcano is increasing, meaning the eruption is imminent," the scientist, Kristanto, said.

Volcanologist Agus Budianto, meanwhile, said that the crater's temperature was among indicators used to predict an eruption and that others, such as vulcanic tremors and deformation of the volcano, were also increasing.

"We registered the centre of quakes was moving closer to the surface, from five kilometres to less than one kilometre (beneath the crater's floor)," he told AFP.

However Budianto said the volcano would only blow out when those were accompanied by big and lengthy tremors.

"In the 1990 eruption, there were non-stop vulcanic tremors within volcano registered for more than one hour prior to an eruption," he said.

Kelut produced continuous tremors on October 19 but they stopped after almost an hour.

"At that time, the energy released from the volcano was not strong enough to push the magma out from the volcano," he said.

Geologists have said the eruption would be comprised of "heat clouds" consisting of searing gases and volcanic debris rushing down the slopes, similar to the most recent eruption in 1990 that left 34 dead.

Indonesia sits on the Pacific "Ring of Fire," where continental plates collide, causing frequent seismic and volcanic activity. The archipelago nation is home to 129 active volcanoes, including 21 on Java alone.

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