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(BBC NEWS) Residents were filmed fleeing a giant ash cloud from Mt Semeru.
Witnesses described nearby villages covered in debris, and thick smoke blotting out the Sun, leaving them in pitch darkness.
The deputy chief of Lumajang district put the number of injured at 41, saying they had suffered burns.
Indah Masdar called for helicopters to help rescue at least 10 people trapped in buildings.
“We’re in big distress,” she said. “It’s harrowing, their families are all crying.”
Indonesia’s disaster mitigation agency (BNPB) later said it knew of 35 people being treated at local medical facilities.
Evacuation efforts have been hampered by choking smoke, a power blackout, and rainstorms during the eruption which turned the debris into mud.
Thoriqul Haq, a local official, told Reuters that a road and bridge from the area to the nearby city of Malang had been severed.
“This has been a very pressing, rapid condition since it erupted,” he said.
Several hundred people have been moved to shelters or left for safer areas, local broadcaster tvOne quoted him as saying.
The eruption took place at about 14:30 local time (07:30 GMT). Local authorities have set up a restricted zone within 5km (3 miles) from the crater.
Airlines have been warned of an ash cloud rising up to 15,000m (50,000 ft).
The Volcanic Ash Advisory Center (VAAC) in Darwin, Australia said the ash appeared to have detached from the summit and was drifting south-west over the Indian Ocean.
The VAAC provides advice to the aviation industry about the location and movement of potentially hazardous volcanic ash.
Campbell Biggs, a meteorologist at the VAAC, told the BBC that the 15,000m plume was higher than the cruising altitude for most aircraft and would cause most flights in the vicinity to divert their flight paths to avoid it.
Ash that solidifies on cooler parts of plane engines can disrupt airflow, which can lead to engines stalling or failing completely.
It also affects visibility for the pilots and can affect air quality in the cabin – making oxygen masks a necessity.
Mt Semeru was quite an active volcano that regularly spewed ash up to about 4,300m, so Saturday’s eruption was a “pretty significant increase in intensity”, Mr Biggs said.
What does volcanic ash do to planes?
The ash cloud should slowly dissipate, he said.
Mt Semeru rises 3,676m above sea level and previously erupted last December, forcing thousands of residents to seek shelter.
It is among Indonesia’s nearly 130 active volcanoes.
Indonesia is on the Pacific “Ring of Fire” where continental plates meet, causing frequent volcanic and seismic activity.
Videos shared by emergency officials and local media showed residents running away as a giant ash cloud rose behind them.