Rescuers have dug out the body of a 13-year-old boy with their bare hands, as improved weather conditions allowed them to resume their search for bodies after the highest volcano on the Indonesian island of Java erupted with fury.

At least 22 people have been confirmed dead and 27 others are still missing after Mount Semeru in Lumajang district in East Java province spewed thick columns of ash as high as 12,000 metres into the sky in a sudden eruption on Saturday triggered by heavy rain.

Villages and nearby towns were blanketed by tonnes of volcanic debris.

Searing gas raced down the sides of the mountain, smothering entire villages and killing or seriously burning those caught in its path.

National Disaster Mitigation Agency spokesperson Abdul Muhari said 56 people had been hospitalised, mostly with burns. He said rescuers were still searching for 27 villagers reported missing.

Nearly 3000 houses and 38 schools were damaged, Mr Muhari said.

The body of the 13-year-old boy was recovered in the worst-hit village of Sumberwuluh on Monday, where houses were buried to their rooftops and cars were submerged.

Crumpled roofs, charred carcasses of cattle and broken chairs covered in grey ash and soot dotted the smouldering landscape.

Also on Monday, rescuers retrieved two more bodies in Sumberwuluh village and five others in Sapiturang, among the worst-hit villages in the Lumajang district.

Search and rescue efforts were temporarily suspended on Sunday afternoon because of fears that heavy rain would cause more hot ash and debris to fall from the crater.

The eruption of the 3676-metre mountain eased pressure that had been building under a lava dome in the crater. But experts warned that the dome could further collapse, causing an avalanche of blistering gas and debris trapped beneath it.

Thousands of displaced people escaped to makeshift emergency shelters after Saturday’s powerful eruption. But many others defied official warnings and chose to remain in their homes to tend livestock and protect properties.

The Lumajang district administration has declared a month-long emergency until January 3.

Semeru, also known as Mahameru, has erupted many times in the past 200 years. Still, as on many of the 129 volcanoes monitored in Indonesia, tens of thousands of people live on its fertile slopes. It last erupted in January, with no casualties.

In another part of Java island, Mount Merapi on Monday spewed smoke and ash high into the air and sent streams of lava and gasses down its slopes for up to two kilometres. No casualties were reported.

Indonesia, an archipelago of more than 270 million people, is prone to earthquakes and volcanic activity because it sits along the Pacific “Ring of Fire”, a horseshoe-shaped series of fault lines.

-AAP