Scientists monitoring the site said tremors were still being recorded while the temperature in Mount Kelud's crater lake had risen sharply in 24 hours.
Surono, one of the scientists, said the spike meant that magma was close to breaking through to the surface, possibly sending a torrent of mud, ash and rock known as lahar down the mountain.
Scientists say the temperature in Mount [EPA]
Kelud's crater lake had risen sharply
The authorities two weeks ago raised the alert status to maximum after Mount Kelud, one of hundreds of active volcanoes dotting the Indonesian archipelago, started showing increased activity.
Despite the imminent threat, however, the authorities said 25,000 people were ignoring evacuation orders and remained in the danger zone.
For weeks, authorities have pleaded with villagers to move to tent camps or government buildings, but have faced resistance.
Many people insist on staying behind to tend crops or look after their homes.
"They are being very foolish," said Sigit Raharjo, a spokesman for the local government in the area. "All we can do is ask them to leave."
Superstitious villagers believe that if they turn off the lights and do not speak ill or loudly, the spirit of the volcano will calm down
Officials declared the zone around the peak as off-limits but there was no attempt to stop people from travelling inside the zone.
Superstitious villagers believe that if they turn off the lights and do not speak ill or loudly, the spirit of the volcano will calm down.
"If I live in a shelter, I can't make money," said Buhirin, 74, a farmer living well within the danger zone.
"I have the courage to stay because I have experienced three Kelud eruptions. I know where the lahar will flow."
Rusdi, a villager from nearby Sempu, said he would wait "until the mountain erupts and then I will seek refuge".